SES-2004 : Add Namespaces.wrap(Set<Namespace>)

This method can be used to insert the Set<Namespace> into API calls that
require Map<String, String>, especially in cases where the API needs to
insert new entries into the Map.
SES-2004 : Start implementing tests for Namespaces.wrap
SES-2004 : Implement all tests for Namespaces.wrap
SES-2004 : Add initial StatementCollectorTest
SES-2004 : Use Namespaces.wrap in ContextStatementCollector
SES-2004 : Test new StatementCollector Model namespace integration
SES-2004 : Remove break for the corner case where there are multiples

If there are multiple prefixes with the same key, as is possible in the
Set given that we specify equality for NamespaceImpl using both the
prefix and name, then this will remove both, as expected for a call to
Map.remove(key)
SES-2004 : Reinstate OpenRDFUtil.verifyContextNotNull on constructor

The constructors were previously chained, but now that they are not, the
check needs to be on the newly independent constructor
SES-2004 : Fix author on StatementCollectorTest
Merged in SES-2004 (pull request #241)

SES-2004 : Populate Model.getNamespaces using StatementCollector