Allow strings with backslash characters

[mwiebe]

The regex matching strings don't accept backslashes. Places to update this include:

Grammar specification:
https://github.com/ContinuumIO/datashape/blob/master/docs/source/grammar.rst#the-datashape-grammar

Lexer:
https://github.com/ContinuumIO/datashape/blob/master/datashape/lexer.py#L41

Tests