remove doctrine1 dependency

symfony 1 has always worked fine without Doctrine (e.g. with Propel, or even without any ORM).
Also, without any stable tag in lexpress/doctrine1, it's impossible to install without lowering minimum-stability to "dev".
add requires
update changelog about dependencies
Merge pull request #35 from garak/patch-1

Remove doctrine1 dependency
Fixbug, refact. BC on sfCacheSessionStorage
Merge pull request #44 from BaptN/master

Fixbug, refact. BC on sfCacheSessionStorage