Change rtags-ac.el usage of `split-string`s TRIM

Hi,

Since some (older?) emacs appear to be missing the TRIM functionality of `split-string` I changed it to use `replace-regexp-in-string` on the beginning and end of the tag string instead.

The goal was to take a tag:  "int foo( const std::string& s, bool enabled )"
and create an arglist: '( "const std::string& s", "bool enabled" )

Then format the arglist for insertion through yasnippet (if available).

Feedback welcome

Thanks!