Fix statusline highlighting after leaving a cmdwin

Because vim doesn't trigger any autocmds after leaving a cmdwin the
statusline in the window the user returned to from the cmdwin would be
highlighted as non-current even if it should have been current.

This issue is resolved by storing the last current window ID in a script
variable, and when leaving the cmdwin we show the last current window as
current instead of detecting it after the WinEnter autocmd has been
triggered (which doesn't happen after leaving a cmdwin).

Closes #184.
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.
Merge branch 'develop' of https://github.com/Lokaltog/powerline into develop