simple snippet caching and possibilty to disable pass-through of POST parameters

Besides the trivial fixes this pull request contains three commits I want to describe more detailed:

**implemented snippet caching**
I'm using the getCmsSnippet() method in the shop main navigation so it is called on every shop request. To avoid unnecessary calls to the underlaying CMS I implemented a simple cache parameter. For a more detailed description of the behavior have a look at README.md.

**added possibility to disable pass-through of POST parameters**
In my case the underlaying CMS is not directly reachable. To reduce possible attack vectors against the CMS I added the possibility to disable pass-through of POST parameters.

**fixed backend issue when clicking on TOXID settings tab**
I ran into an issue where the tab navigation in the TOXID settings dialog changes to the default `shop_list` view:
![toxid-backend-demo](https://cloud.githubusercontent.com/assets/149483/5216611/51e01896-7636-11e4-8402-bee4b67502b0.gif)
By fixing this issue via blocks instead of copying the `shop_list.tpl` the differences between OXID editions remain untouched.