Support for Mobile SDK 2.0.x

Here's my implementation of support for Mobile SDK 2.0.x. Do note that it features breaking changes. Since preconnectWithEnvironment and initializeWithClientIdsForEnvironments are required in 2.0.x, I combined the corresponding methods in the PhoneGap plugin. In addition, I added support for future payments but didn't have the means to test it very thoroughly.

I also cleaned up parts of the .js documentation, but there's still some work to do there. Due to inconsistencies in syntax, I couldn't quite decipher which documentation tool the comments were originally written for, but I'd be happy to fix them to conform to, say, YUIDoc.