(#122) - Profile support (custom, core & complete)

Adds the following options:
- profile
- profileDiff
 - include
 - exclude

This allows users to specify exactly which parts of express-pouchdb need to be loaded. Dependencies are handled by a check to ``app.includes[partThatsNeeded]``, often abstracted away via ``utils.requires``.

The order in which the routes are loaded is specified by index.js.

See for examples on how to use the included tests. The second argument ('opts') needs to be documented in README.md though.
The PouchDB test suite needs some valid response at /_session. The full session support is quite heavy (in terms of amount of code), so this is a fallback that's enabled in the core profile.