Some additions to root pom, to help with Release build

[kdvolder]

Changes:

1. Adds a property 'p2.qualifier' which is to be set to something like 'SNAPSHOT' or 'RELEASE'.
   The default value for the property is 'SNAPSHOT'.
2. A 'release' profile is defined to set the property to 'RELEASE'. The profile is activated by adding '-Prelease' to the mvn commandline. (It is almost as easy to just set the property on the commandline: '-Dp2.qualifier=RELEASE' directly. If you prefer this then you could omit the profile definition from the pom.
3. configure packaging plugin to incorporate 'p2.qualifier' into the qualifier of all built-artefacts thereby making it easy to see whether a pluing/feature version is a release or not.
4. enable 'archiveSite' option in packaging plugin. The result of that is that a zipped update site is created in 'org.dadacoalition.yedit.update/target/site_assembly.zip'. Might be convenient to upload the site somewhere for publication and/or archival.