adding support for appengine

App Engine cannot use http.DefaultClient, so this makes TwitterApi use TwitterApi.HttpClient instead of http.DefaultClient (still defaults to http.DefaultClient).

To use anaconda in appengine:

```
import (
  "anaconda"
  "appengine"
  "appengine/urlfetch
)
func get(writer http.ResponseWriter, request *http.Request) {
  api = anaconda.NewTwitterApi(...)
  api.HttpClient = urlfetch.Client(appengine.NewContext(request))
}
```