implemented for caching the laterpay_post_teaser as default

* implemented for caching the laterpay_post_teaser as default
* added nonces to purchased post and statistic tab ajax
* renamend LaterPay_Controller_Post_Content to LaterPay_Controller_Post
* added new lpVars for better JS-handling
* added "use strict"; to js
* jslint check

references to issue: #258
`exit` will kill the PHP interpreter outright. I can't imagine that's really what we want to do here and *if we do* I would like at least a comment on why this is the right choice