Autoload improvements.

Refactored autoload into two files to make proper use of autoload advantages.

Autoload is useful as it saves having to load too much code on startup.
However, calling the functions in the autoload/ file from the plugin/ file
results in the load-on-startup happening anyway, so it might as well be in
plugin/.  This patch splits the autoload code into two files:

* EasyMotion/Init.vim for the stuff that's loaded on startup and
* EasyMotion/EasyMotion.vim for the stuff that's loaded on demand.