Issue #783: python3 compatible - remove relative imports.

This commit starts a series of changes to eliminate relative and redundant imports.

- do not import theano.tests unless theano.test() function is called.
- reuse  TheanoConfigParser instance from configdefaults.
PEP 8 compliance changes.
Avoid importing theano into itself.
Removed unused imports.
Use absolute imports.
Use absolute imports.
Use absolute imports.
Use absolute imports.
Removed unused imports.
Removed unused imports and converted relative imports to absolute.
Converted relative imports to absolute.
Converted relative imports to absolute.
Converted relative imports to absolute.
Removed unused import.
Removed unused import and converted relative imports to absolute.
Removed unused import and converted relative imports to absolute.
Converted relative imports to absolute.
Removed unnecessary imports.
Replaced relative import with absolute.
PEP8 reformat.