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
Merge pull request #253 from cpennington/field-data-as-service

Access FieldData as a Runtime service
Make tests pass without deprecation warnings
Merge pull request #257 from cpennington/remove-warnings

Make tests pass without deprecation warnings
Fix all pylint errors except TODOs
Make exceptions moved to xblock.exceptions still available in xblock.core
Merge pull request #260 from cpennington/clean-up-pylint

Fix all pylint errors except TODOs
Merge pull request #259 from cpennington/backcompat-exceptions

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
Merge pull request #264 from cpennington/default-pre-pylint-number

Make violation checking a little more automatic
Merge pull request #265 from cpennington/fix-coverage-script

Fix script/coverage