When bootstrapping fails to setup `g:_uspy` (e.g. without Python),
the functions used in the autocommands are not being defined.
Since it makes no sense to have autocommands defined for UltiSnips in this
case, this patch deletes them (via a new augroup 'UltiSnips').
Removed parsing responsibilities from SnippetManager and instead put
them into the new module providers. Renamed private methods on
SnippetManager that are not meant to be called by external libraries to
start with _. Refactored tests so that expected failures can be tested
and therefore removed the testing flag from SnippetManager.
If a python module tried to load UltiSnips without the autoload file
being run first, UltiSnips_Manager was not defined. This broke YCM and
maybe others too. This fixes
https://github.com/Valloric/YouCompleteMe/issues/820.
