Py-liant

Functional changes:
- Implicit filters now apply even for primary key URLs
- Implicit filters can be defined using a callable

Internal changes:
- fix test requirements
- restructure README.md

Functional changes:
- allow polymorphic casting in relationship drilldown
- added support for implicit (default) filters
- added support for loading hints profiles
- loading hints: use - operator to unload collection
- loading hints: use + operator to join-load collection

Internal changes:
- documentation update

Functional changes:
- added _isnull autofilter
- added polymorphic casting in CatchallView route syntax
- added polymorphic loading hints

Fixes:
- fix exception when drilling into non-dynamic relationships

Internal changes:
- documentation updates and fixes
- unit testing coverage