[WIP] get_share() to retrieve share information about known share (by id)

@PVince81: Any comments about this? (#7) Because I don't know if it's a right approach :wink: 
I'd say make PublicShare inherit from ShareInfo and move share_id to ShareInfo.

Also ShareInfo should have getters like "get_id()", "get_type()" (for the share type), "get_owner()", etc, similar to FileInfo but for shares and for all the information that exists for any basic share.
So ShareInfo is the base class for any shares and PublicShare is a special case when the type is 3 (public share)