Put the memory manager last, if it is found.

This means that it is more likely that we will pick a plugin that is actually
relevant to real-world usage.
Allow overriding default QContactManagerEngine via env var

This patch adds support for overriding the default
QContactManagerEngine via the QTCONTACTS_MANAGER_OVERRIDE environment
variable.
Merge pull request #2 from rburchell/master

Improvements to manager selection.