[FEATURE] Context view helpers

With this view-helpers you can use the [`TYPO3_CONTEXT`](http://docs.typo3.org/typo3cms/CoreApiReference/ApiOverview/Bootstrapping/Index.html#bootstrapping-context) to output different content for your development,testing or production environment.

Please don´t merge until the following questions are solved:

1. `TYPO3_CONTEXT` [got added in 6.2](http://docs.typo3.org/typo3cms/CoreApiReference/singlehtml/#what-s-new) but vhs emconf says 6.0 is supported - how do we deal with that old versions? Can we have view-helpers that only work with 6.2?
2. I created this view-helpers in the namespace `ViewHelpers/Context/*`. I used the wording "Context" because the core uses it, although "Environment" would feel more intuitive to me. There are also already some view-helpers in a similar namespace: `ViewHelpers/Condition/Context/*`. I am open for suggestions to improve this situation.
Missing line break here between file header and imports; here and in following classes. Note to self: needs a php code sniffer sniff!