Improve Unite.vim integration

Hi. First of all, thanks for this wonderful plugin. I found this several weeks ago and I'm already addicted to.
But I found that the integration to Unite.vim is not strong enough. I could not yank, delete, annotate the bookmark within the Unite interface thus I decied to send this PR. 

With this PR, you can

- Add new Unite source called 'vim_bookmarks' (while Unite.vim has 'bookmark' source in default) which can
    - Delete the selected bookmarks
    - Yank path of the selected bookmarks
    - Yank content of the selected bookmarks
    - Yank annotation of the selected bookmarks

Belows are working screenshots of the new 'vim_bookmarks' source.

![2014-11-20 0 40 50](https://cloud.githubusercontent.com/assets/546312/5108619/e785b3bc-704e-11e4-9722-092a161b0ef3.png)
![2014-11-20 0 41 01](https://cloud.githubusercontent.com/assets/546312/5108620/ea25f10e-704e-11e4-86cd-9e6f77ca612a.png)

I would love to add "Annotate the selected bookmark" but it required `s:refresh_line_numbers` in `plugins/bookmark.vim` thus I couldn't (I don't want to move these private method to autoload while you might not like that strategy). So I'll really be happy if you add that feature or if you decied to move several utility functions to autoload.

Please check it out and feel free to change my modification, especially the documents :-)
Shouldn't this be rather put in [bm_sign.vim](https://github.com/MattesGroeger/vim-bookmarks/blob/master/autoload/bm_sign.vim)?