Fix omego version packaging and add version command

This PR ports some version fixes from openmicroscopy/snoopycrimecop#102 into the `omego` package.

- the version number is saved under `omego/RELEASE-VERSION`
- the RELEASE-VERSION file should be packaged as part of the source distribution
- add a new `omego version` command to get the version number

To test this PR
- tag this repository (since no tag has been pushed to origin so far)
  ```
git tag 0.1.0 888e7da -m "Tag version 0.1.0"
```

- build a source distribution and check everything is copied at the right place in the tar ball
  ```
python setup.py sdist
```

- create a virtualenv, activate it, install omego and test `omego version` is working properly (inside and outside of the Git repository)
  ```
$ ~/virtualenv/bin/virtualenv ../venv
$ source ../venv/bin/activate
$ python setup.py install
$ omego version
$ (cd .. && omego version)
```

As part of this PR we should consider tagging the main repository so that this command does not throw an error on origin/master.