add support for unsigned int values import/export (close #2)
Fixes #4

Retry the command (one time only) if the connection was lost
Fixes #5

Reimplement runsource to reset the cache in case of SyntaxError

Behaviour of runsource based on:
http://docs.python.org/2/library/code.html
Call RosShell runcode instead of parent class
Rename ros_import into ros_publish, and ros_export into ros_subscribe.

The old names are kept for now, but will disappear in the
future.
Merge remote-tracking branch 'upstream/master'