Document manipulation: Implement remaining (missing) ReQL commands

[tbolier]

Issue summary

The current ReQL document manipulation commands implementation is currently missing in the driver.
https://www.rethinkdb.com/api/java/

Document manipulation

• pluck
• without
• merge
• append
• prepend
• difference
• setInsert
• setUnion
• setIntersection
• setDifference
• () (bracket)
• getField
• hasFields
• insertAt
• spliceAt
• deleteAt
• changeAt
• keys
• values
• literal
• object

The following sections can be removed only, whenever they are not applicable or you have a simple question.

Actual Behavior

• Not implemented

Expected Behavior

• A working implementation including unit / integration test.

[Th3Mouk]

getField is already implemented