JCR-2113: Writing access control

This patch addresses the following as indicated in https://issues.apache.org/jira/browse/JCR-2113:
a. adjust jcr2dav module and make sure the RepositoryConfig gets a valid ac-provider configuration parameter set
b. complete setPolicy, Batch#setTree, + testing
c. complete removePolicy for jcr2spi and partial implementation for removal of protected items on jcr-server.

@anchela @dbu : this is wrong after coming across this from the JcrRemotingServlet API docs. it says the following:

NOTE the following special handling of JCR properties of type
* Binary, Name, Path, Date and Reference:
* the JSON value must be missing
* the POST request is expected to contain extra multipart(s) or request
parameter(s) for the property value(s)
the content type of the extra parts/params must reflect the propertytype: "jcr
value/" + PropertyType.nameFromValue(Property.getType).toLowerCase()

The key point is the second one which says that the JSON value must be missing. For access control this corresponds to the rep:privileges mv property which has a Name type.. so it should be: rep:privileges : [] and not rep:privileges : ["jcr:read"]... 

i am fixing this now;)