Pretty HATEOAS URLs by stripping regexes out. Closes #466.
Properly serialize nullable floats and integers.

Serialization of floats and integers was silently failing on null (None)
values. This was subtle because the field would still end up being
stored as a null.

Closes #469.
Merge branch '469' into develop

Conflicts:
	CHANGES
Projections fix for when 'allow_unknown' is True.

When 'allow_unknown' is True:

- Client projections don't crash.
- Datasource projections are not ignored.

Closes #497.
Support for multiple MongoDB instances.

- 'mongo_prefix' resource setting (eg. 'MONGO1', 'MONGO2')
- 'app.data.mongo_prefix' overrides resource level 'mongo_prefix' and can
  be used by custom Auth classes to invoke user reserved,isolated DB instances
- both MongoDB and GridFS instances are cached to preserve connection pools,
  TCP connection, etc.
- 'eve.io.base.DataLayer' and 'eve.io.media.MediaStorage' interfaces are
  untouched so other engines should not be influenced by these changes (not
  breaking backward compatibility)

All tests are passing but multi-db tests are still missing. Documentation still
missing (will probably also need a tutorial for user-reserved db instances).

WIP.