Merge branch 'master' into develop
Use lein instead of maven, deploy to clojars
llow dynamic selection of the method of comm w/vbox.

The vmfest provider now can cake a new parameter `:vbox-comm` to
select the method of communication of Pallet with the VirtualBox
subsystem. The options are :xpcom for XPCOM, and :ws for Web Services.
It defaults to :xpcom.

There is no longer the need to add either vboxjxws or vboxjxpcom jars
to the classpath prior to using pallet-vmfest, in order to select the
communication method. Pallet-vmfest contains both jars and one of them
will be injected in the classpath dynamically depending on the value
of `:vbox-comm`.

NOTE: Selecting the communication methods with VirtualBox can only be
done once per JVM session.
Update README to reflect new dependencies.
Ignore stopped/inaccessible VMs

If a VM is misconfigured, or stopped, pallet cannot manage it. From
now on these VMs will be effectively ignored. For example, if you have
a `vm-test-0` VM that is stopped, and you converge to 1 the same
group, you will get another VM in this group. Converging back to 0
will remove this new vm, leaving the stopped `vm-test-0` alone.

Similarly, all VMs created by pallet are in the `vmfest` group in
VirtualBox, and pallet will not _see_ any VM outside of this group.
The VMs outside the pallet group were already ignored, since they
don't have any group tag, but now they will not even be
considered (e.g. not listed in pallet.repl/show-nodes).
Improve reliability of dynamic loading of vboxj*.jar into classpath.

Forward p.c.vmfest/add-image to p.c.vmfest.service/add-image. This
function used to belong to p.c.vmfest. Also, referencing this function
does not trigger the loading of p.c.vmfest.service, which would break
at compile time since vboxj*.jar gets loaded at runtime when
configuring the first vmfest provider.

Prevent the loading of p.c.vmfest.service if no vboxj*.jar lib is in
the classpath. Will result in exception if you try.
Forward all image functions to p.c.v.service.
Remove use of slingshot
Remove ws and xpcom profiles
Update tests for vbox-comm classloading
Add release script
Updated project.clj, release notes and readme for 0.3.0-alpha.4
Merge branch 'release/0.3.0-alpha.4'