Split up which for Unix and Windows

[maxmeyer]

Unix and Windows need a separate logic to find executables in PATH. This splits up the original implementation and uses Aruba.platform to decide which implementation to use.

[maxmeyer]

We now only need to make sure, that the logic is correct on windows.

[maxmeyer]

@weedySeaDragon I took your comments about the #match? into account. Now there's a default-class which just fails. All other classes use a matcher which checks specific things.

[maxmeyer]

Ashley, would you mind to have a look into the windows implementation? It is still failing, but Appveyor-builds took too long to use them for fixing bugs.

[weedySeaDragon]

I see 2 things immediately:

1. The full path to cmd.exe is not necessary. (Windows will always know where to find cmd.exe). So WindowsCommandString.initialize can be changed to: __setobj__ format('cmd.exe /c "%s"', cmd)
2. You are only passing 1 argument to ChildProcess (the entire command string). In Windows, ChildProcess requires you to pass each of the string components as a separate argument:

process = ChildProcess.build("cmd.exe", "/c", "path\to\command\cat")

(also see the first comment in #283)

[maxmeyer]

1. The full path to cmd.exe is not necessary.

For now: Unfortunately yes... The current code is not wrapped with with_environment. This is part of another PR of mine... This will be removed if the PR is merged.

1. You are only passing 1 argument to ChildProcess (the entire command string). In Windows,

Oh. Will fix this.

Thanks.