Add pyporject.toml file to specify build-system requirements

As of v10 of `pip` PEP 518 is supported. This allows for a 
`pyproject.toml` file to be created that specifies any build-system 
requirements that need to be installed *before* the `setup.py` file is 
run.  This solves the issue with pip needing `cython` and `numpy` before 
they are installed.

This has been tested with the command `pip install .` in clean python 
envrionments (no packages installed) in both python 3.6 and 3.7.  Pip 
was able to install the package and all dependcies in the correct order.

Author: CKrawczyk

pyproject.toml

@@ -0,0 +1,7 @@
+[build-system]
+requires = [
+  "setuptools",
+  "wheel",
+  "cython",
+  "numpy"
+]