Pod::Podfile::TargetDefinition.

* If no workspace is specified, then the basename of the default
  target's project will be used.
* If no workspace is specified and multiple projects have been
  specified (in several target definitions), an Informative error
  will be raised indicating that the user should specify one
  explicitely.

NOTE: The workspace code doesn’t actually exist yet, it’s the last thing to do in this branch.

I don't think this is the place to be asserting the path behavior anyway,
so I'm now just verifying that the build var gets set.

This will allow more flexible paths where the won't have
to type the path to the install command each time.

Assists in getting a fresh working copy up and running
with a single command.

Currently initializes & updates all git submodules

Remove ability to specify link_with and platform through TargetDefintion options, for now. As Platform takes an options hash too, this was becoming a bit messy.