Removed now unused option argument_mapping

This option will be removed in favour of <Plug> mappings
Trying to fix empty argument list bug
Return to old position when no matching parenthesis is found
Removed debug
Fixed bug with trailing whitespace

When operating on an function argument list consisting only of
whitespace, the closing parenthesis would be selected for the operator.
Noe we just select all the whitespace.
Fixing empty function arguments bug

Operating on () was buggy as hell before. Now the behaviour is more
consistent. Since we can not select nothing, visual mode just returns.
'c' now goes inside the  parenthesis and 'd' just goes to the closing
one.
Merge pull request #2 from UnrealQuester/master

Minor bugfixes