Bring NTriples parser in line with the W3C NTriples Proposed Recommendation

This was brought about by my suspicion that the NTriples parser was reacting badly to non-ASCII literal characters. For example, take http://users.ox.ac.ac.uk/~kebl2765/salary.nt and run:

    $ rdfpipe -i nt salary.nt

The '£' characters have been mangled into 'Â£'.

This pull request:

* enforces that by the time `unquote` is called, we're using `unicode`/`str`, not `str`/`bytes`
* adds the \b and \f escape sequences
* removes the \/ escape sequence
* always validates when unquoting
* removes the use of the `unicode-escape` codec, as it doesn't deal with non-ASCII literals correctly in this context

I've not added any more tests, but all current tests still pass.