Home directory resolution system

[ezzatron]

It's a common use case for paths to use ~ or ~user to indicate home directories. Pathogen should have a system for dealing with these cases.

Some things that probably need to happen:

1. FileSystemPathFactoryInterface includes methods to get the current working directory, and system temporary directory. It probably needs to be expanded to have methods to get the current home directory, and the home directory of a specified user. They should always internally use the 'platform' path factory - this just ensures that they are Windows paths on Windows, and Unix-style on anything else.
2. A new resolver at FileSystem\Resolver\HomeDirectoryResolver that implements the newly refactored PathResolverInterface and uses an instance of PlatformFileSystemPathFactory to produce home directory paths using the new methods created in 1. This resolver could probably cache home directory lookups, I assume they're unlikely to change at runtime.

This task could be tackled at the same time as #36.

[ezzatron]

@darianbrown This task is pretty much ready to roll; I've finished my refactorings for now. Let me know if you're no longer interested though. Also, if you get on a roll, #36 is a pretty similar task!