Move development dependencies into Gemfile
Move "Contributing" section into dedicated document for better GitHub integration

See https://github.com/blog/1184-contributing-guidelines.
Test against Ruby 2.0.0
Convert to Ruby 1.9 hash syntax
Enable SSL verification

Closes #18.
Add bundler development dependency
require 'mtgox/version'

Because require hashes based on the string, not the expanded path of the
file, it's important that we require the version file the same way that
it will be required elsewhere to avoid constant warnings.
Use of custom objects is not necessary here
Add cryptographic signature
Don't hardcode my home directory
Don't implicitly require simplecov
Specify minimum rspec version
Add gem version badge
Add gem signing instructions
Rename public certificate
Remove non-existant CHANGELOG from gemspec
Bump version to 0.8.1
Add official support for Ruby 2.0.0 [ci skip]
Reword [ci skip]
Use relative links [ci skip]