Install Titanium SDK if user does not have one installed

[tonylukasavage]

We check to make sure the user is logged in, but we make the assumption that they have a TiSDK installed. As triple may become the first way a developer interacts with Titanium, we should make this experience transparent.

1. Make sure the developer is logged in. If they are, skip to step 3. If they are not, pop up a message that directs them to https://my.appcelerator.com/auth/signup and tell them to sign up.
2. Once signed up, tell them to execute ti login
3. Before creating or launching the repl app, use ti sdk to ensure that the developer has a Titanium SDK installed. If they do, skip to step 5.
4. If they don't have an SDK installed, run ti sdk install to get the latest stable version installed.
5. Check to make sure the selected TiSDK is greater than or equal to 3.2.3. Print a warning message if it is not.
6. continue as we have before

We'll also probably want to note the other requirements somewhere, like XCode and such, but I haven't really thought about how far I want to go in recreating the install experience. I think I might want to just keep it to the Titanium SDK.