Merge pull request #95 from Murrx/search-filter

Basic implementation of activity search
Fixed dynamic fields serializer for drf 3.0.2
some style cleaning
Merge pull request #96 from vplme/fields-fix

Fixed dynamic fields serializer for drf 3.0.2
Merge pull request #97 from Murrx/clean-search-backend

some style cleaning
ActivityFilter added
BasicFilter, BasicFilterBackend
Test filters added