Fix gemspec and readme

[cokernel]

A couple of minor fixes:

• The gemspec was invalid, and the Gemfile still used 'http://rubygems.org' as a source.
• The link to the PairTree spec was broken.
• The method Pairtree::Identifier.decode did not roundtrip properly in Ruby 2.0.

[aelkiss]

Superceded by #5