Add plans and jetpack-modules endpoints

[retrofox]

This PR adds:

WPCom#plansList()

import wpcomFactory from 'wpcom';

wpcom.plansList()
  .then( list => console.log( list ) )
  .catch( err => console.log( 'oh, no!' ) );

WPCom#plansFeatures()

Site#plansList()

Site#jetpackModules()

Site#jetpackModule( moduleId )

SiteJetPackModule#get()

SiteJetPackModule#update()

SiteJetPackModule#activate()

SiteJetPackModule#deactivate()

Testing

> make test wpcom.plans
> make test wpcom.site.plans

[timmyc]

* @return {Promise} Promise is returned correct?

[retrofox]

👍

[timmyc]

Some minor documentation and formatting items listed above. Also a comment about why you have tests for the same endpoint under 3 different versions - just more curious than anything :)

[retrofox]

thanks @timmyc