Fixed initialization of Occi::Core::Links w/ strs

`convert` in Occi::Core::Links now correctly transforms strings
into Occi::Core::Link instances. The last part of any URI/URL
is taken as `link.id` and the whole URI/URL as `link.location`.
Improved `get_by_location` on Occi::Collection

`get_by_location` can now resolve instance locations into
category locations and return appropriate categories.
Merge pull request #76 from arax/link_coversion_fix

Bug fixes related to link parsing and look-up on collections