Ported argtextobj.vim plugin

This plugin provides a text-object `a` (argument).
See http://www.vim.org/scripts/script.php?script_id=2699
Updated version to match unstable update site
Updated argtextobj core version to match unstable update site
Added support for single argument function calls/declarations.
Added counter support to argtextobj.

If counter given it selects **N** following arguments.
Throw an exception instead of selecting the whole text in argtextobj.
Fixed single argument and empty strings in argtextobj.
Merge pull request #237 from igrekster/argtextobj

Ported argtextobj.vim plugin