Issue #12: Add support for new document collection API

[nadre]

• adding DocumentCollection class
• adding list document collections

[DavidHuebner]

Added new class DocumentCollection that holds information about a document collection and is the access point for some API endpoint (delete(), get_number_of_documents(), import_documents())

• Added support for five new endpoints
  1. Getting list of all DocumentCollection
  2. Create a new DocumentCollection
  3. Getting information about a single DocumentCollection (mainly used to access the number of documents)
  4. Deleting a DocumentCollection
  5. Importing a plain/text or solr-xml document into a DocumentCollection
• New tests in test_client to test these functions on the client level and some specific tests in test_document_collection
• Added an example solr-xml