Teach list_snippets_dict to escape apostrophes in the description.

This was discovered while trying to use
UltiSnips_SnippetsInCurrentScope() with some snippets that have
apostrophes in the description (such as printf('...', ...)).
Merge pull request #94 from jszakmeister/fix-escaping-in-list_snippets_dict

Escape apostrophes in the description inside of list_snippets_dict.
Merge remote-tracking branch 'upstream/master'