Merge pull request #54 from shk3/subtitle_fix

Subtitle fix
PEP-8: Fix spaces before colon.
PEP-8: Fix more whitespace issues.
PEP-8: One last pass of whitespace fixes.
PEP-8: Remove trailing semicolon from end of statements.
Fix #58.
edx-dl: Fix appending to strings. Closes #58.

Python strings are immutable and they have no way of being "appended" or
having things appended to them.
Fix indentation for consistency.

Indentation in Python should always be consistent, as the language semantics
depend on it.
Add blank lines for legibility.

These lines mark some logical sections of the file and it is good to have
such sections both visually and mentally separate.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
Separate imports by origin.

We follow PEP-8 here and put all the standard library imports together,
separated from (and followed by) the 3rd party modules.

There is more work along these lines to deal with Python 2/3
compatibility.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
cosmetic: Python doesn't need boolean conditions enclosed by parens.

This makes the code less cluttered: "all things being equal, fewer pixels
make for easier reading".

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
Merge pull request #59 from shk3/fixes-for-master

Merge changes to improve the quality of the code with
respect to PEP-8.
Store the subtitles from edX to the destination directory rather than the working directory to fix a bug in #53 and #43 .