Move xblock-family mixins into a separate python module
Extract Runtime-services machinery from XBlock into a mixin
Make XBlocks and Fields access FieldData via a runtime service.

Maintains the ability to pass a FieldData in XBlock constructors for
backwards compatability.

[PLAT-206]
Deprecate passing field_data through the runtime constructor
Make tests pass without deprecation warnings
Fix all pylint errors except TODOs
Make exceptions moved to xblock.exceptions still available in xblock.core
Pull XML serialization out into a separate mixin [PLAT-229]
Make the number of pylint violations a default value in the script, overrideable elsewhere
Fix script/coverage
Add a pre-push hook that can be used to validate pylint and pep8 values
Add methods to IdReader and IdGenerator for working with XBlockAside usage ids [PLAT-210]
Make base abstract methods raise NotImplementedError
Ignore common non-coverable lines
Use python-versioneer to prep PyPI pushes

[PLAT-279]
Add a makefile for running tests/packaging
Make travis.yml use the make file
Don't bother checking coverage of setup.py
Associate aside view handlers with xblock view they decorate

[PLAT-208]
[PLAT-209]
[PLAT-233]

Co-Author: Don Mitchell <dmitchell@edx.org>
Init Plugin class vars per subclass