[2.1] Edited JMSSecurityExtraBundle doc link Edit

It now points to the real documentation of this bundle and the version used in Symfony2.1, which is 1.2.
Update reference/constraints/Image.rst

Seems that annotations do not support trailing commas in parameter lists like PHP arrays do.
[Dependency Injection] small changes
Added failure_path param
Replace MinLength with Length
replace maxlength constraints with length equivalents

Conflicts:
	reference/constraints/Collection.rst
max_length can also be guessed from Length
Added a note about request accept-* headers parsing.
Added information about accept-* headers request methods.
Created intro text of the Twig Reference
Added functions section
Added filter section
Created tags section
Added 'almost core' bundles
Added missing dot in property path assembly.
Adding link to how to render templates
Adding link to @ParamConverter
Changed uses of we in security cookbooks
Removed space around dot operator.
[Book/installation] Fixed "image file not readable" warning