npm modularized

Hiya,

I'm trying to switch appium over to including all dependencies as npm modules. Currently, selendroid is a git submodule which we build using a shell script.

In this PR I've forked selendroid and added a package.json file so that it can be published to npm. There's a `prepublish` script which compiles all targets using maven, so that appium can just require the module and get a path to a compiled selendroid.apk.

Do you want to manage/maintain or control this npm module? I'll just keep merging in changes for each version of selendroid, and publishing, but in case you want control over it I'm happy to hand it all over and such.