Reinstate Maven uploads

Here's an attempt to re-instate the Maven uploads. You [wrote](http://markmail.org/message/jgd7ohxu3naeukj7):

> my attempts to introduce the Saxon version number to the artifact names were apparently unsuccessful

This PR sets the POM version to the value of the property `mvn-version` which in turn is based on `saxon-version`. It results in artifact file names like `xmlcalabash-1.0.12a-94`. Is that OK ?

Note that I also included a fix to the bnd config file (used for the OSGi metadata generation); it now looks at the `saxon95` property to declare the correct version of the Saxon dependency.