Documentation improvements for 2.0.1.

- Explain TableQuery macro in schemas chapter (#621)
- Fixed some typos
- Add findBy to migration guide (#655)
- Clarify use of .tupled in migration guide (#656)
- Explain schema names in schemas chapter (#659)

Fixes issues #621, #626, #655, #656, #659.
Enable documentation tests on Travis CI
Merge pull request #671 from slick/tmp/docs-2.0.1

Documentation improvements for 2.0.1.