Map config paths onto Packages

TODO: Move this logic into PathSource
Move package mapping to Source
Progress towards compiling with dependencies
Use the resolver to collect packages
Compiling with deps progress
Merge branch 'master' of https://github.com/carlhuda/cargo
Let the resolver return a PackageSet
cargo-compile delegates to cargo-rustc
Bring up to date with Rust master
Start breaking up cargo-compile
Fix a bunch of warnings
Cleanup ProcessBuilder, work towards getting tests passing
Break apart internal and CLI errors

The Cargo library now uses internal errors to represent problems, and
the operations convert those errors into CLIErrors, which contain a
user-friendly error message, additional information for verbose mode,
and an exit code.
Add some convenience methods
Core struct refactor
Comment cleanup
Source list returns summaries
Test unit tests passing again
Clean up error messages from config
Merge pull request #7 from Aeyk/patch-1

Update README.md