Added support for Zepto and bug fixes

Added support for Zepto. Now the Baasbox JS client works also if the
user is using Zepto as JavaScript library. The main update is that if
the user is using Zepto, then:
1) the information about the current user being logged in the app is
stored in local storage, rather than using Cookies,
2) the initial setup of Ajax headers is done using Zepto’s function,
rather than jQuery’s.

Other minor bug fixes, mainly concerning typos and JS syntax.