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
thanks a lot! Seems that something went missing. Re-added the check here: https://github.com/laterpay/laterpay-wordpress-plugin/commit/638e37afc2cdd3747cdb56d37574ec441a45be97

we've to check when:

* cache is activated + post is purchaseable + visitor has purchased post -> load content and replace teaser
* cache is activated + post is puchaseable + admin turns preview mode "admin" on -> load content and replace teaser

:)