queue: use msg.Release where we used PutBack
app: move and unexport queue messages

These constants are not used in other packages anymore. Also, I moved
the declaration to the queue.go file, which makes more sense.
queue: better wording in Handler.loop docs
provision/juju: move access key and secret key declaration on tests

Other tests should depend on these values too.
provision/juju: added new handling function for queue messages

This function will handle the action "add-unit-to-lb", also defined in
the same file.

This message will have at least two arguments: the name of the app and
all units. For example: []string{"myapp", "myapp/0", "myapp/1",
"myapp/2"} will add three units ("myapp/0", "myapp/1" and "myapp/2") to
the load balancer named "myapp".

Related to #247.
provision: smaller interfaces

The Named interface removes some duplication and allow the code to
handle stuff by its name only (load balancing on Juju requires only the
name of the app, for example).
provision/juju: simplify code related to ELB

Use provision.Named instead of provision.App. Smaller interfaces for the
win :-)
provision/juju: make ELBManager.{Register,Deregister} handle N units

Handling a variadic number of units makes more sense, and also avoids
multiple calls to Amazon Web Services (Register and Deregister calls
already support a variadic number of instances within AWS).

Related to #247 and #250.
provision/juju: use variadic capability from ELBManager.Register

When handling add-unit-to-lb message from the queue, add all units at
once :-)

Related to #247.
provision/juju: change assertion, so it's more reliable

Sometimes the units come out of order. Sorting before the assertion.