fixed comment to make golint happy.
added cli pkg, added airspace cli impl.

added a new cli pkg holding all the cli command implementations (was in
main.go). added the implementation of the airspace-ls command, for now
instantiating a soaringweb instance directly (flexibility will come with
the config PR).
renamed ls to get cmds, added better docs.
added airspace cli, add ctx mgmt, added mock.

added cli implementation for the airspace-get command.

in addition adds functionality to manage an app Context obj,
which is for now only used in the cli.

separate plugin management into its own package (was in common).

added a mock package for dummy implementations of all interfaces (useful
for testing).