FIX: function names should not begin with 'l:'

Vim 7.4.260 disallows the use of colons in function names other than those
which start with 's:'.
(https://groups.google.com/forum/#!topic/vim_dev/M4_E1QhXVxA)

Fixes #8
(https://github.com/amdt/sunset/issues/8)