ValueError instead of wrong windows.

Closes #93.
Merge pull request #94 from mwtoews/master

Change behaviour of `masked=None` for read()
Release GIL around GDALRasterIO.

Also add Python 3.4 example of using thread pool executor from
concurrent.futures.
Add more comments to the example.
Add concurrent processing doc.

Also release GIL when warping, but this has no speedup in practice.
Make that 3x.
Buh bye to the old parallel example.
More comments in the new async-rasterio.py example.
Explanation of from whence the speedup comes.
Apply PEP8 formatting guidelines for .py and interpreted to .pyx, .pyd
 - Remove or comment-out unused imports
 - Remove trailing whitespace, and other stylistic consistencies
 - Max 80 chars per line, spaces between operators, etc.
 - Fix a few minor typos; change ValueError to consistently use NULL
Change behaviour of logging
 - rasterio logger is set up once in __init__, and shared throughout
 - GDAL logger remains in _drivers.pyx as before
 - Both rasterio and GDAL loggers don't show any messages by default,
   but have been set up to be enabled by starting Python up showing
   all options, or "python -W all"