Track extconf.rb and nokogumbo.c

These files shouldn't be ignored. Fix gitignore to track them.
Only look for 'lib/nokogiri' files

Gem.find_files can return other directories as well as the lib dir when
searching for Nokogiri's files, which can cause install to fail.

Filter out any non-lib results from the list.
Merge pull request #3 from mattwildig/fix-install

Fix install