Enable text-domains for translations

Enables translation / label definition over text-domains. 

*Benefits:*

* Could help to tidy-up the current language files by using contexts.
* Allows multiple use of label-keys depending on the context:
 * names.ini;  libraryA = "Name of library A"
 * links.ini; libraryA = "Link to library A"


*How to:*

* A Subfolder in the languages folder defines  a text-domain.

 ```
  languages/date/en.ini
 ```
* Definition of text-domains in config.ini:

 ```
 [TextDomains]
 textDomains[] = date
 ```
* Use with transEsc view-helper:

 ```php
 $this->transExc('date::day_placeholder')
 ```

*Comments*
* Current label handling stays the same.
* Alternatives to the '::'-notation would have been:
 * Fourth paramter in transEsc view-helper. But that would be a bit unhandy.
 * Separate view-helper or direct use of the zendTranslator view-helper (would exclude escaping).