Mixpanel for Haxe

This library contains the interface to Mixpanel API for Haxe/OpenFL.

Based on mixpanel-as3.

Differences from the AS3 version:

1. passing a null token to the constructor will block all sendRequest operations. This can be useful if you want to simply disable Mixpanel but not have tons of branch code.

2. library send POST isntead of GET requests by defaut. OpenFL legacy has a bug where URLLoader ignores GET CGI data. This may be reversed in the future, but there are no real implications for user side.

3. only SharedObject is supported for backend storage, and OpenFL cares about the abstractions.

4. The library identifies itself to Mixpanel as mp_lib=haxe, following Mixpanel standards.

Everything else should be the same and you can follow AS3 usage documentation.