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
Added support for json-encoded strings for Dict field
Addressed review notes. Removed pretty printing as it created more problem than value