Make comparators compatible with RDF-1.1

In RDF-1.1, all Literals will have a datatype.

This means that comparators must check language before datatype when determining equality, as the previous method of checking datatype would return equals for unequal language tagged literals.

RDF-1.1 starts with Sesame-2.8, but adding a patch and tests for the issue now should make it easier to switch over

Some of the tests will start to fail when using Sesame-2.8, as some assume that plain literals are different to xsd:string typed literals, but these tests should be marked. In addition, the sort order for xsd:string typed literals and language literals may also change, causing those tests to trivially fail.

NOTE: Travis is failing due to the removal of the rdfjson module. The fix for that is in pull request #35