Stash manager

As discussed in [GPRESOURCES-213](http://jira.grails.org/browse/GPRESOURCES-213) and [pull request #44](https://github.com/grails-plugins/grails-resources/pull/44), I externalized the stashing functionality to a StashManager class. As there was no real need for Groovy and this will also be used very often, I implemented everything in pure Java.

There are also unit and integration tests for the new code.

If there is anything, which should be changed, just tell me.