added USER and WORKDIR keywords
Merge pull request #1 from spikegrobstein/add_user_workdir_keywords

added USER and WORKDIR keywords
Dockerfile commands are case-insensitive
Line must begin with # to be a comment (ignoring whitespace)
Keywords/commands are also only valid at the beginning of the line (barring whitespace)
Merge pull request #2 from tianon/patch-1

Better Specification/Implementation Conformity
Add note about docker contrib