add support for ldap

Signed-off-by: Maël Lavault <mael.lavault@mailz.org>
add psql shortcut for database url FIXES #12
Merge branch 'master' of github.com:moimael/django-environ into moimael-master
add tests for ldap protocol
Merge branch 'moimael-master'
Improve README and version to 0.3.1