We don't need to use Tempfile anyway, we can stick the tarball in the pod root and then delete it.

Generally speaking, this should be much faster than cloning a repository (even a shallow one)
and we can take take advantage of Github's tarball support. I've made this opt-in (using the
:download_only option) in case this breaks any existing functionality people expect (e.g. 
using submodules).

Started adding some specs for the target installer, including for this fix.

because previously each TargetInstaller would resolve it's specs separately.

Instead, we resolve everything in one go as before, but have the resolver return a hash of
target dependencies to resolved specifications, so when the target installers are run, we only
install the pods for that target that should be in there.

Likewise, we should only call the post-install hook of a specification with the target installers
it belongs to.

for an external source can be loaded from the sandbox, because it's already been downloaded, then
just use it rather than trying to pre-download it again.

Conflicts:
	Rakefile

Renamed method argument to make it clear it takes a podfile or a spec.

External encoding set to UTF-8 on Ruby 1.9 to fix crash caused by non-ascii characters in pod description

External encoding set to UTF-8 on Ruby 1.9 to fix crash caused by non-ascii characters in pod description