(#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.
Here is defined what files a profile consists of: custom by default doesn't include anything at all; core is as close to pouchdb-express-router as I could get, pouchdb-complete contains everything.