Update entity_provider.rst

Fixed Fatal Error for UsernameNotFoundException
Merge pull request #279 from lucabartoli/patch-3

Update entity_provider.rst
Merge pull request #287 from gregquat/origin/2.3

updating from symfony-docs 2.3
Merging #290
Merging #292
Merging #292
Merging #293
Merging #294
Merging #294
Merging #295
Merging #296
Merging #297
Merging #298 + fixing typos
Fixing conf
Merging #300 (Spartaaa)
Merging #301
Merging #302
Fixed link to session doc

The Session section was referring to a re-documentation to be done with Symfony version 2.1 (!). I just fixed it by providing the link to the actual session documentation, doing the exact same thing as the english doc.
Merge pull request #303 from gnugat/patch-1

Fixed link to session doc
Update asset_management.rst