Issue #783: python3 compatible - remove unnecessary imports.

This commit is the first step of the plan to make Theano imports compatible with 3.x:

1. Remove unnecessary imports.

2. Convert relative imports to absolute.

3. Fix circular dependency issues by either eliminating "from xyz" imports or moving them inside functions.
Merge pull request #1218 from abalkin/remove-unused-imports

Issue #783: python3 compatible - remove unnecessary imports.