Add Python 3.4 to tox environments
Add CORS headers for all files, not just fonts

It transpires there are things other than fonts which can break when you
start serving your static files from a different domain, and the W3C
explicitly states that this header is safe for publicly accessible
content. See: http://www.w3.org/TR/cors/#security
Use FILE_CHARSET setting for default charset
Slightly refactor cache header code
Rename variable for clarity
Add Content-Length header

This is particularly important in preventing partial content getting
cached if the connection drops mid-request.
Remove option to disable gzip serving behaviour

It was needless extra configuration: there's no good reason to disable
this and if someone really wants to they can create a sub-class and
override the `find_gzipped_alternatives` method.
Move immutable file handling into WhiteNoise base

Of course it's still up to the Django sub-class to identify which files
are immutable, but we want as much of the behaviour as possible in base
so it's available to other potential sub-classes.
Refactoring
Better HTTP compliance

Return 405 responses for methods other than GET/HEAD.

Return no body for HEAD requests.
Don't nest main application call in except clause

If the application itself then raises an exception this results in
confusing tracebacks on Python 3. (Thanks to @dstufft for pointing this
out.)

Closes #8
Start reworking docs
Use read-the-docs theme locally
More docs