API calls to filter user list by username/email

add pagination to /api/users

add support of id based filtering

add num_pages field to users api output

add tests for paging data and ability to parse page_size from request

using ListAPIView and DRF filters

Disallow unfiltered lists

Updated docstrings
Merge pull request #63 from edx-solutions/ziafazal/add-filters-to-user-api

Ziafazal/add filters to user api