feat(ParamConverter): add named ParamConvert

this allow the PropelParamConvert to be used by name "propel"
Priority also lowered from maximum 10 to 1 #246
Check if there's a _route request attribute before getting it

Apparently Symfony doesn't add a _route parameter to subrequests (esi render call) so PropelParamConverter throw an error accessing getOption method on not-existent object on line 98.
Merge pull request #248 from andrea-chris/patch-1

Check if there's a _route request attribute before getting it
Merge pull request #247 from jaugustin/feat-named-converter

feat(ParamConverter): add named ParamConverter
Allow Symfony >2.1 in the 1.2 branch
Merge pull request #250 from ClementGautier/symfony-2-3

Allow Symfony >2.1 in the 1.2 branch
Merge branch '1.2' into survos