Merge pull request #3 from jwhitlock/bad_request

Handle 400 Bad Request w/ JSON API Error format
Preparing to work on the parser
Add basic support for links when parsing
Added support for M2M in links
Switch creation test to use links
Fixed README formatting
Add mixin for JSON API partial PUT
Render null ForeignKey fields
Add test for forward ManyToManyField
Handle PrimaryKeyRelatedFields