render_markdown_from_file() was using a cStringIO as the file

output, which wasn't compatible with the results from

markdownFromFile() on Python 3. This wasn't caught due to the lack of

unit tests for this function.
Now, the stream is written to a io.BytesIO. Missing documentation was

added that specifies that the input stream must be a byte stream.
Along with this, the code has been made safer from leaks by closing the

outgoing stream in a finally: clause. While this shouldn't technically

be necessary (garbage collection should handle closing this for us), it

is slightly more correct.
A unit test was added to ensure this works on Python 2 and 3.

Unit tests pass on Python 2 and 3.
Without the fix, the test fails on Python 3 .