add code coverage

I think it will be nice to add functionality of [simplecov gem](https://github.com/colszowka/simplecov).
After running
```
bundle exec rake
```
Open httparty/coverage.index.html and you will see something like this ![test coverage](http://oi62.tinypic.com/4tui2p.jpg)
Notice:
[Why .simplecov](https://github.com/colszowka/simplecov#using-simplecov-for-centralized-config)
In .simplecov use [profiler](https://github.com/colszowka/simplecov#profiles) 'test_frameworks', which defined [here](https://github.com/colszowka/simplecov/blob/master/lib/simplecov/defaults.rb#L11).