Make it work with phonegap buid

To fix issue #96.

Copied code from "poiuytrez/AndroidInAppBilling" which allow that the Android-Billing-Code can be given not just via file also as a parameter.
(Should be fine both plugins use MIT license)

It should then be possible to install it like this:
>> cordova plugin add cc.fovea.cordova.purchase --variable BILLING_KEY="MIIB...AQAB"
>> phonegap local plugin add cc.fovea.cordova.purchase --variable BILLING_KEY="MIIB...AQAB"

That will then automatically create the file.