Some tweaks and more love for callbacks

[niess]

• Running 0.4.2 on Ubuntu 14.04 LTS I got encoding errors with html codes, e.g. α. I didn't manage to escape those by changing my settings. However, forcing the encoding to utf8 in docorium.rb, as discussed here solved the problem.
• Functions returning an enum were not reported in the Returned by field of the corresponding enum page, because exact matching was not allowed. The corresponding regexp was changed.
• A callback entry has been added as a submenu of Types, in the sidebar. Furthermore if a menu or submenu is empty it is no more reported on the sidebar.
• In addition to functions, callbacks are also reported as Argument in or Returned by for enums and structs.
• The Returned by and Argument in items are comma separated to ease reading.
• This is a nice tool :) Exactly what I was looking for for my current project: lighter than Doxygen, C+git dedicated.

[carlosmn]

Thanks. I'm not sure about the 'callbacks' name for function pointers, since they're not always callbacks as such, but you might be providing an interface in a struct. But I'll merge as-is and then I'll see if I feel strongly enough to change it.

[niess]

Hello :) One could flag function pointers as callbacks only if the end with _cb, _cb_t. Though it would enforce some coding convention :(