Update cache.py

Added support for TASTYPIE_DEFAULT_FORMATS. Assumes TASTYPIE_DEFAULT_FORMATS[0] if no format is specified and TASTYPIE_DEFAULT_FORMATS exists.
Update cache.py

Removed unnecessary DEFAULT_FORMAT override.
Reverting valid_cache_format() logic

Set it back the way it was (but as a one-liner that hopefully clarifies the intent for dummies like me).
Merge pull request #1 from tclancy/master

Support TASTYPIE_DEFAULT_FORMATS