Add a hook so additional scripts can be run after creating VM

Allow additional scripts to be run as part of the OMERO Virtualbox build.

For example, to include OMERO.searcher as part of the VM (note this is using the manual-scc-merge-all branch for convenience, and assumes you can build the standard VM in the first place):
```
PARENTDIR=$PWD
git clone git@github.com:openmicroscopy/omero_searcher.git
cd omero_searcher
git checkout origin/manual-scc-merge-all
```
or alternatively just download `setup_omero_searcher.sh` from
https://github.com/manics/omero_searcher/blob/omero_searcher_vm-11240/docs/vm/setup_omero_searcher.sh

Then in the openmicroscopy checkout:
```
export OMERO_POST_INSTALL_SCRIPTS=$PARENTDIR/omero_searcher/docs/vm/setup_omero_searcher.sh
./build.py build-vm
```

If you're lucky you'll now have a VM image with OMERO.searcher pre-installed and configured. Import some images, calculate some features, and search :).

See also
http://trac.openmicroscopy.org.uk/ome/ticket/11240
https://github.com/openmicroscopy/omero_searcher/pull/12

Note the actual OMERO.searcher VM setup script `setup_omero_searcher.sh` is a bit of a mess (or rather quite a big mess) due to the ancient pip version which is missing a lot of functionality. There's also the question of where it should go- its not part of the core omero distribution, but it is specific to the OME build infrastructure so is the omero_searcher repo the right place for it?