Improve scaladoc comments and clean up API for Slick 2.1.

- Lots of new and improved doc comments.

- Deprecate Query.where and restrict Query.filter and Query.filterNot
  to only accept Columns. Desugaring of for-comprehensions will always
  generate calls to withFilter which still allows plain Booleans.
Deprecate more operators:

- Deprecate "is" and "isNot" operators for possible reuse in a later
  version. Fixes #751.

- Implement "isDefined", "isEmpty" and "nonEmpty" for Column[Option[_]],
  deprecating "isNull" and "isNotNull" (see #750).
Merge pull request #741 from slick/tmp/2.1-scaladocs

Improve scaladoc comments and clean up API for Slick 2.1.