[INTERNAL] add independent part of context package

These classes have few dependencies and are added first
The call to Saros.checkInitialized() in SarosPluginContext is disabled
until the class Saros is added to this repository

Change-Id: I091d1d464db43265cb6e2a087f10ca2dea0d8c44
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1698
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>