Add optional docset bundles support

The docset support requires the libdocset library to be installed. The
latest version of the docset library is currently available here:
https://github.com/roman-kashitsyn/libdocset

Unfortunately, the library isn't included in any PPA yet. I think it will be convenient to include it to the gnome3-team PPA. I'm new to debian package distribution so I appreciate any help.

If the library is not installed, the DevHelp builds normally without docset
support.

Screenshots:
![devhelp-docset-home-view](https://f.cloud.github.com/assets/847968/2062854/ad77396a-8ca2-11e3-820f-b250916e1774.png)
![devhelp-docset-search-view](https://f.cloud.github.com/assets/847968/2062855/ad91917a-8ca2-11e3-9b49-3d0ab20ce440.png)

Note: no .po files are updated in this commit.