Add some license headers snippets

I have improved upon the approach from https://bugs.launchpad.net/ultisnips/+bug/883952 and now make use of the comments extractor machinery in all.snippets to make this language independent.

In order to fill the copyright line automatically, the snippets assume that the following two global variables have been defined by the user:
g:snips_author
g:snips_author_email

Beware, I do not know Python at all, so it may be suboptimal to say the least...