Allow overriding session cookie key with environment variable
Allow overriding session cookie key with environment variable

Needed for sessions to work when hosting 2 gitorious instances, one at
example.org, second at second.example.org.

Gitorious-backup/mainline,SamuelMoraesF/Gitorious,SamuelMoraesF/Gitorious,gitorious/mainline,spencerwp/mainline,gitorious/mainline,elcom/gitorious,Gitorious-backup/mainline,SamuelMoraesF/Gitorious,Gitorious-backup/mainline,tigefa4u/gitorious-mainline,spencerwp/mainline,gitorious/mainline,rae/gitorious-mainline,elcom/gitorious,spencerwp/mainline,Gitorious-backup/mainline,spencerwp/mainline,elcom/gitorious,rae/gitorious-mainline,tigefa4u/gitorious-mainline,rae/gitorious-mainline,gitorious/mainline,tigefa4u/gitorious-mainline,tigefa4u/gitorious-mainline,rae/gitorious-mainline,SamuelMoraesF/Gitorious,elcom/gitorious