Add Track charge API method

This little patch adds the new "$append" or track_charge() API call to the gem. It uses the People API, but $append behaves a bit different from the increment/set API calls so I had to hack around a few abstracted things.

Also, I noticed there is already an `append` method in the Mixpanel Tracker class, which makes tracking charges using an `append` method impossible. I have chosen to name this new method to `append_charge` in stead, which does break consistency with the Mixpanel API.

I propose renaming the original `append` method that appends an action to the queue in a new pull request. This would break all apps currently making use of it.