Refactor plugin, bump to version 2.0.0

Main changes are:
* Browserify plugin
* Hide internals by providing a public API object
* Calling svgPanZoom returns an API instance 
* Added onPan and getPan methods
* Added getZoom method

It breaks backward compatibility with previous version of the plugin. Also it is not possible to take first SVG Element from document, you should specifically indicate SVG Element selector.