Code changes to support SQLAlchemy 0.9

Apart from some version checking, the only change has been the use of
`sqlalchemy.orm.properties.RelationshipProperty` instead of
`sqlalchemy.orm.properties.PropertyLoader`.

Apparently `PropertyLoader` was renamed to `RelationProperty` in SA
0.5 [1] (!), which in turn got renamed to `RelationshipProperty`.

[1] https://github.com/zzzeek/sqlalchemy/blob/1abd53a3556b9593d9eba868d69c13bae3c3a7ab/doc/build/changelog/changelog_05.rst
Merge pull request #12 from okfn/sqlalchemy-0.9-support

Code changes to support SQLAlchemy 0.9