Use filter() in ShowAvailableSnips

Fixes #103
Remove redundant undo point creation

This was originally removed in 824f902 but somehow got back in. As
stated in 824f902, the use of <C-G>u creates an undo point
unconditionally. Also this closes #37.
moving the repository list to snipmate-snippets - having one central place is eassier to maintain
adding Adnan Zafar to lists of maintainers - he is doing most work now
snipmate-snippets is vim-snippets because it will contain snipmate and UltiSnip snippets
Add radicalbit to list of contributors
Remove most select mode compatibility (?) mappings
Use <Plug> mappings

This deprecates g:snips_trigger_key(_backwards).
Reposition the cursor after updating vars

This is needed should a var appears in the same line but prior to the
current tab stop.
Avoid updating variables after calling self.remove().

This appears to fix a bug where all variables would be cleared when
exiting insert mode then opening a line below the current line.
Merge pull request #120 from hpesoj/master

Fix disappearing variables bug
Add hpesoj to list of contributors
Move contributors to separate file
Overhaul README