Fix EGit Tycho build

Fix EGit Tycho build

- Add missing pom-first dependencies as described in [1] to fix the build.
- Create EGit update site.
- The build of the update site assumes that egit and jgit are checked out
  under a common root folder, this is needed to refer to the JGit update
  site. If the layout is different the URL pointing to the JGit update site
  has to be passed to the maven build as system property jgit-site, e.g.
     mvn -Djgit-site=file:/C:/tmp/site clean install
  or
     mvn -Djgit-site=http://point.to.jgit/site clean install
- Introduce property for Eclipse p2 repository to prepare builds against
  other versions of Eclipse.
- Adding information about mailing lists, issue tracker and license.

This change depends on the following JGit change :
- Ie4026f15f6250c4933dccf6f31b5009b90c036bc

[1] https://docs.sonatype.org/display/TYCHO/Dependency+on+pom-first+artifacts
    BUT: since Tycho 0.6.0 target-platform-configuration has
         groupId org.sonatype.tycho !

Change-Id: Idf90bd4a29c36013f582c8f99424c8fc72597bc5
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>