Added parent POM `plugin-profiles` for common plug-in settings

For building the plug-in you now need to pass 3 profiles:

* eclipse-indigo or eclipse-juno: To decide whether the build is targeting
  Eclipse Indigo or Eclipse Juno.

* scala-2.10.x: To set the Scala version. The only currently supported profile
  is the one for Scala 2.10.

* scala-ide-stable, scala-ide-dev or scala-ide-nightly: For the Scala IDE
  version that will be used as a targeted installation for the build.

The ./build.sh script is now the simplest way to build the scala-search plug-in.
The default build targets Eclipse Indigo and the latest Scala IDE stable release for
Scala 2.10.