Minor fixes to nquads handling

[mielvds]

This PR does minor fixes to the nquad parsings/serialization, including

• improving the regex for extracting bnodes (this makes urdna2015#test059 pass)
• using stdlib line splitting instead of custom regex
• isolating escaping and unescaping literal values for fixing encoding and escaping issues later on (~ urdna2015#test060$)
• fix a KeyError in N-Quads serialization (see Fix a KeyError in N-Quads normalization #52)

(Sidenote about escaping and urdna2015#test060: this is a very tricky one - believe me, I tried- and I think this whole file should eventually be replaced by RDFLib instead of pursuing this)