Have proper Lexer/Parser

[xlr-8]

Hi,

Regarding issues #67 & #86 I thought it could be good to integrate a more linux-like lexer/parser (with BNF and stuff), to avoid those small & painful issues.

Some relevant links - reading before bed:
http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html
http://www.teicm.gr/icd/staff/nikolaid/learning_the_bash_shell.pdf [page 327 and more]

Keep up the good work!