Allow to read configured keys

Currently, there is no way to read the content
of the Git Configuration in a  way that would
allow to list all configured values generically.
This change extends the Config class in such a
way as to being able to get a list of sections and
to get a list of names for any given section or
subsection.
This is required in able to implement proper
configuration handling in EGit (show all the
content of a given configuration similar to 
"git config -l").

Change-Id: Idd4bc47be18ed0e36b11be8c23c9c707159dc830
Signed-off-by: Mathias Kinzler <mathias.kinzler@sap.com>