Add an advanced text generator based on markov chains.

The advanced text generator's text()-generator is compatible to
Lorem's one and can be used as a drop in replacement.
Fix several issues with markov chain Text provider
Add de_DE locale for Text provider
Fix typo in de_DE/Text locale
Improve documentation of the Text provider
Add Text provider to Factory
Update Text provider with the latest suggestions
Fix tests for Text provider
Merge pull request #254 from TimWolla/advancedTextProvider

Add an advanced text generator based on markov chains.