Ensure we can run on 5.2
Actually add namespace.php

Props @willmot
HM_Backdrop_Server and HM_Backdrop_Task should be loaded from root namespace
Limit the key length so we avoid going over the transient name limit
Reduce key length down to 28 so that it also works on multisite
Merge pull request #5 from humanmade/fix-fivetwo-compat

5.2 Compat branch should work on 5.3
Merge branch 'key-length' into five-two-compat
Merge branch 'five-two-compat' of github.com:humanmade/Backdrop into five-two-compat
Hardcode the complete namespace as __NAMESPACE__ is null here
use call_user_func as $class::method() fatals on 5.2
Merge pull request #4 from humanmade/five-two-compat

Ensure we can run on 5.2