Improved the Ruby Gem to make it usable from a Rakefile

I wanted to use GitVersion from my Rakefile like this:

```ruby
require 'git_version'

include GitVersion

task :default do
  puts git_version.sha
end
```

I extended the gem to also include the lib folder and made a parser that calls the executable (once) and makes all the properties from the JSON available via `method_missing`. You may use `snake_case` or `CamelCase` to access the JSON properties.