set sensible comments and commenstring

This (re-)enters commentstring and comments
settings for erlang.
The default erlang-ftplugin already sets these, but when
using vimerl those settings seem to be ignored.

Here are the original settings:
https://code.google.com/p/vim/source/browse/runtime/ftplugin/erlang.vim#43
Merge pull request #19 from 0robustus1/master

set sensible comments and commenstring