WIP: Support absolute/relative and file/directory types

[rofinn]

Summary

• Introduce Form and Kind concepts (though not exported) which paths are parameterized by.
• Aliases for dispatching (e.g., AbsolutePath, RelativePath, FilePath, DirectoryPath)
• Stricter parsing based on the above types (e.g., directories must end with a separator)
• Dropping ispathtype in favour of just tryparse
• Wrapper methods to return the appropriate types
• Extra tests for the new parsing approach
• Update AWSS3.jl
• Add support to FTPClient.jl?

Closes #72, #78 and #79

[rofinn]

I'm not sure if it's worth continuing to work on this. This approach has some nice properties, but it adds a lot of complexity and hacks that may confuse users coming from the Base.Filesystem API.