Update android plugin to Google Analytics V2, and update to CordovaPlugin structure

n.b. There are some incompatabilities between setVariable and the new setCustomMetric and setCustomDimension. The new methods have been added for future support in GAPlugin.js, and setVariable implemented with setDimension (not sure if this is appropriate, but trackPage and trackEvent do work now).
Merge pull request #3 from MarkPaxton/master

Update Android to Google Analytics V2