Support prefix moving

[jasonkarns]

We would like to have definitions like latest, nightly, lts, current, etc which pull down the "latest" respective version. Ideally, we would like the installed version to be named precisely per the version number as having an installed version named latest is hardly useful after a few days.

• perhaps attempt to mv the version in after_install_package hook, accounting for PREFIX_PATH (see Revert "After install, rename nightly to the actual version string" #236 )
• perhaps attempt to modify the PREFIX_PATH as part of the definitions "main" run, rather than in hooks?
• perhaps we need to expose a hook in node-build that a definition can invoke to force/override the installed package name?

Questions to resolve:

1. What to do when a given architecture is not included in the specified "latest" build? Should it build latest from source? Or get a slightly older version that does have a binary build? This would require checking platform in the build definition itself.
2. What to do when the "latest" is already installed? The pre-install check that node-build performs is based on the build definition filename. This will be insufficient if the version name isn't known until the definition is run.

closes #145