Revert "Added new cmdline option --parallel-request."

This reverts commit 01e0724330b9557d18ba0c1f454151d5860fbf2a.

As it turned out, this feature has some bugs not covered by tests.
Added Plugin.version[T] method.
Changed return type of PluginWithDependencies.dependsOn.

No a new case class PluginDependencies is returned. So, we can later add additional constraints, like .e.g. version ranges.
Changed PluginDependency from case class to trait.
Implemented plugin validation based on plugin dependencies.
Moved implementation of PluginHandle.

Moved implementation of PluginHandle into PluginAwareImpl and made it thus non public. With that, also the PluginAware trait got a cleanup.
Improved equals method and made some fields more private
Imported and Scala-ified OSGiVersionRange from OSGi core 5.0.

The orginal version was Java code under same license (Apache 2.0). It was reworked in Scala but is untested.