Change function names to adhere stricter rules

Since v7.4.260 there were some changes done to be more strict about what
can be used in function names. This change makes Vdebug follow these new
rules.

This resolves #155

Signed-off-by: BlackEagle <ike.devolder@gmail.com>
Merge remote-tracking branch 'bie/fix-function-names-no-longer-allowed-since-vim-7-4-260' into fh2