Improve vim plugin loading

Several of the plugin loading methods have been joined into one plugin
file that will be loaded by updating the runtimepath. More informative
error messages will be displayed if Python support is missing or if the
module import fails.

Note that this commit will break existing plugin loading, the new
method with updating the runtimepath will be required.

Closes #156.
Closes #181.
Add tips & tricks to the docs

Closes #182.
Fix math error in humanize_bytes

Closes #175.
Fix vim plugin path in Arch Linux packages
Add note about zpython branch

TODO: To be removed after (if) zsh maintainers accept the patch.

Refs #104.
Add modified_buffers segment

This segment returns a comma-separated list of modified buffers. The
segment is not enabled by default.

Closes #173.
Use functools.wraps on memoize decorator
Update sphinx configuration
Work around missing vim module when generating sphinx docs

Horrible solution, should be fixed asap.
Add docs for all segments
Merge branch 'develop' into local-develop