Better install behavior for packages not in snapshot

[snoyberg]

• stack install foo should default to whatever's in the snapshot, and if that's not available, choose the latest in the index (reporting as such to the user)
• To force a specific version of a package to be installed, can run stack install foo-1.2.3
• For ease of use, can provide stack install foo:latest to ignore whatever's in the snapshot
• Provide a command stack upgrade which is a synonym for stack install stack:latest

To be especially useful, we should complete #59 as well

[snoyberg]

I'm going to leave off that last bullet for now, and we can just document the upgrade process elsewhere later.