Separate "browser" target into web.browser/cordova

Cordova projects often have a different set of files than web targets,
so we would like to be able to target different client architectures in
our bundles. Ideally, we allow the user to use arbitrary client
architectures - but this patch is a step in the right direction by
abstracting out more of the hard coded "browser"/"os" lines.

We accomplish this separation in a backwards compatible way by allowing
api.___ commands to target a "client" architecture. For example,
api.addFiles('a.js', 'client') adds 'a.js' to both the 'client.browser'
and 'client.cordova' targets.

Effects on 0.9 packaging stuff: packages don't have to change, but the
"data.json" file in ".meteor0" has "browser" in some places. We think we
have to fix the troposphere code where this data.json is created.

Some plugins will also be backwards-incompatible with this change, since
many have a "clientArch.matches("browser")" line in the plugin
code. Ideally, we fix plugins so that this stops being an issue, but for
now package authors can just patch that line.

At the compiled (unipackage) level the new names are 'web.browser' and
'web.cordova', replacing 'browser'. In package.js, the new names are
'client.browser' and 'client.cordova', serving as an adjunct to 'client'.
fix orderKey regexp
Merge branch 'packaging-only-client-archs' into packaging