add username support in --maintainer
Propagate error back to caller.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Merge pull request #134 from rjnagal/master

Propagate error back to caller.
Merge pull request #131 from dotcloud/add_username_maintainer

add username support in --maintainer
add --milestone to issues
display milestone in list
Merge pull request #132 from dotcloud/issues_milestone

add --milestone to issues