Bring site information up to date

- remove references to dead rubyforge links
- update documentation link to rubydoc.info. Fixes #1, https://github.com/ruby-ldap/ruby-net-ldap/issues/107
- update supported rubies >= 1.9.3 (reflected in Travis CI build matrix)
- remove support for non rubygem installs
- update copyright year

cc @mtodd @schaary @ekalber @michaeltwofish @epinault