Replace duplicate code for loading project manifest
auto merge of #253 : jdeseno/cargo/find-root-manifest, r=alexcrichton

A few of the sub-commands seem to use the same code for loading the project manifest.

I'd like to get rid of the nested matches but, I can't figure how to use something like `and_then` & `or_else` where the type is promoted (Option -> CargoResult etc.)