This was my favourite feature, so I'm glad to see it back!

Exa now uses the new IO, Path, and Filesystem libraries that have been out for a while now.

Unfortunately, the new libraries don't *entirely* cover the range of the old libraries just yet: in particular, to become more cross-platform, the data in `UnstableFileStat` isn't available in the Unix `MetadataExt` yet. Much of this is contained in rust-lang/rfcs#1044 (which is due to be implemented in rust-lang/rust#14711), but it's not *entirely* there yet.

As such, this commits a serious loss of functionality: no symlink viewing, no hard links or blocks, or users or groups. Also, some of the code could now be optimised. I just wanted to commit this to sort out most of the 'teething problems' of having a different path system in advance.

Here's an example problem that took ages to fix for you, just because you read this far: when I first got exa to compile, it worked mostly fine, except calling `exa` by itself didn't list the current directory. I traced where the command-line options were being generated, to where files and directories were sorted, to where the threads were spawned... and the problem turned out to be that it was using the full path as the file name, rather than just the last component, and these paths happened to begin with `.`, so it thought they were dotfiles.

Fixes #58, technically.

updated to latest rust nightly

Fixes #55.

Still missing a few Beta features, but it compiles!

- Copy requires Clone
- current_dir returns a Path now
- num_cpus moved to a crate

This module provides feature-specific implementations, and also dummy implementations for when they aren't supported by the system or OS.

Doing it this way limits all the #[cfg(feature)] annotations, as we can now just include the module or not.

- Changes to users crate

- old_path::Path isn't imported by default anymore
- range -> ..

The path_filename function/method isn't declared public. If the declaration is changed, rustc complains about a conflict. This seems to be the minimal change needed for tests to run.

Add "`--group` is useless without `--long`"

Make Git Column Optional

Closes #47

making Exa.load() stat files in parallel