Dsl spec

Created test codes for DSL to fix #9. 

Test Codes. 

1. Create rack_test_json/dsl_spec.rb and httpclient_json/dsl_spec.rb 
2. Fix driver_examples.rb not to use 'capybara/spec/driver' because the file does not exist on capybara 2.0.x. Now test passes in both capybara 1.x and capybara 2.x

Main codes

1. Fixed a bug that DSL `body` does not work on capybara 2
2. Fixed a bug that DSL `status_code` does not exist on capybara 1