Fixed weight ignore issue on grade calculation

[amir-qayyum-khan]

What are the relevant tickets?

fixes #95

Background:

• Add Grade to assignment.
• Expected behaviour: Grader will offer a grade between 0 and 100, and should be multiplied by weight to get % of full problem.
• Actual behaviour: Grade is a grade between 0 and 100. Problem weight seems to be ignored.

What's this PR do?

In this PR i am using weight to calculate grade percentage. Applied this logic on two points. Images are attached to demonstrate.

How should this be manually tested?

• set weight from studio and observe grade

@pdpinch

Screenshots (if appropriate)

[codecov-io]

Codecov Report

Merging #242 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #242      +/-   ##
==========================================
+ Coverage    92.4%   92.46%   +0.05%     
==========================================
  Files          15       15              
  Lines        1594     1606      +12     
  Branches      101      101              
==========================================
+ Hits         1473     1485      +12     
  Misses        111      111              
  Partials       10       10

Impacted Files	Coverage Δ
edx_sga/sga.py	94.03% <100%> (+0.04%)	⬆️
edx_sga/tests/integration_tests.py	100% <100%> (ø)	⬆️

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[amir-qayyum-khan]

fixed atomic block issue in make_student.

Traceback (most recent call last):
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/db/models/base.py", line 973, in delete
    collector.collect([self], keep_parents=keep_parents)
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/db/models/deletion.py", line 222, in collect
    elif sub_objs:
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/db/models/query.py", line 258, in __nonzero__
    return type(self).__bool__(self)
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/db/models/query.py", line 254, in __bool__
    self._fetch_all()
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/db/models/query.py", line 1118, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/db/models/query.py", line 53, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch)
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 899, in execute_sql
    raise original_exception
TransactionManagementError: An error occurred in the current transaction. You can't execute queries until the end of the 'atomic' block.

[pdpinch]

@amir-qayyum-khan can you rebase this and address the merge conflicts?

I've been sitting on this for a while because I'm not certain what the actual requirement is. Once it's rebased I'll run it by someone from the MITx team to get their input.

[amir-qayyum-khan]

rebased @pdpinch