- `user_repr` is now a method
- Fix for python 2.7
- excepts -> expected

Note: This refactor opens the door for implementing a ContextualLexer for Earley.
But unlike the existing one for LALR, it will have to be computed at runtime,
rather than ahead of time.

make _all_terminals a dict
added raw attribute to Pattern
rename nice_print -> user_repr

rename LexerConf.terminals to LexerConf.tokens
Make Exception message generation lazy
Made a few classes new-style

- Refactored lexer interface into LexerConf
- Lexer now compiles regexps only when used (especially useful for ContextualLexer)
- Lexer now doesn't validate on deserialize (noticable speedup)