Added initial logging support.

The idea is that you can log your requests using either the `include` syntax and the direct `HTTParty` class.

Using include:
```ruby
class GoogleFetcher
  include HTTParty
  logger Rails.logger
end
```

Using HTTParty:
```ruby
HTTParty.get "http://google.com", :logger => Rails.logger
```
I have included two log styles, one that resembles apache's access log and another that resembles curl, which can be used by passing `:log_format => :curl`.

You can also control which method is called on logger by passing `:log_level => :debug` for example.

Please let me know if the code does not adhere to the project's standards or if I need to change anything.

Opinions and criticism are welcome.