Drop Ruby 1.8 support.

In June 2013, Ruby 1.8.7 will officially reach end-of-life (EOL) status
meaning that *no* more patches will be released. Additionally, Ruby
Enterprise Edition (REE) has reached EOL. JRuby and Rubinius both have
1.8 and 1.9 modes so there are upgrade paths available here.

Finally, relying on a Ruby version equal or greater to 1.9 eliminates
the multi_json requirement which has started throwing warnings when the
stdlib version of json is being used.

Users requiring Ruby 1.8 can continue to use the 0.1.x gem releases
which remain functional and highly stable.
Fix gemspec.
Swap out multi_json for stdlib json.
Eliminate multi_json from the documentation.
Merge pull request #2 from fnichol/drop-ruby-1.8

Drop Ruby 1.8 support.