Add method for querying repositories

There are several issues left ATM:
- The method SUSEStudio.getRepositories does not properly URL escape
  the parameters before concatenating the HTTP query.
  But that is also missing for other existing methods, such as
  SUSEStudio.cloneApplianceFrom, so if that is an issue, it
  should be applied the same way to other methods as well.
- The performance is rather poor, because the result set of
  the request returns a big number of repositories. This is
  probably due to the use of the simpleframework library, and
  I don't see an easy workaround that can be implemented without
  more specification.
- The Repository model might not be entirely correct, because
  I could not find the API documentation that details all fields,
  instead I only found example xml responses. So I tried also to
  look into the C++ API. What I came up here, is something that makes
  sense for me (without through understanding of SUSE studio).
  Some fields may be missing.
- The <repository> element can have a element <matches>. I don't
  see, how this can be mapped easily with simpleframework. I am
  not familiar with this framework, but it seams that maybe
  org.simpleframework.xml.convert.Converter or
  org.simpleframework.xml.strategy.Visitor is the solution.
  First that would need a clear specification what the XML
  elements exactly mean and how they should represented in
  java model classes. Then we could think about how to deserialize
  it using simpleframework.