Basic support for symbolic links

I'm managing some large files in my git repository using [git-annex](http://git-annex.branchable.com), and it works by creating symbolic links.  Currently, gollum has no support for symlinks; it just returns their targets as a string blob.

This pull request adds initial support for symlinks, just to `File#raw_data` and `Page#raw_data`.  This gets non-page file serving working via symlink, and I think it makes pages work also, though I haven't tested that as thoroughly.  I'm no expert with the gollum code base, so I'm not exactly certain where else in the code might need to be changed to accommodate symlinks in other places.  But this change by itself would be really helpful to me, at least.

I'd be happy to add more commits to this pull request if anybody's interested.  I'm not sure how to write tests against this, as I don't know how to work with the Git repositories in `test/examples`. But I'd be happy to write up some tests if people were interested.