Search for pages instead of gems

Hi there,

this PR makes gem-man search for page names instead of gem names. Previously, if you didn’t know which gem a manpage is contained in (e.g. you had a gem named "utilities" which contains serveral different executables and manpages) you weren’t able to view a commands manpage. Take for example the ronn-format(7) manpage. Whereas previously executing

~~~~~~~~~
$ gem man ronn-format
~~~~~~~~~

would not find anything (you had to know that manpage is contained in the `ronn` gem and hat to issue `gem man ronn`) the very same command now properly displays the ronn-format(7) manpage.

Vale,
Quintus