Add test slaves for Mac and Windows and possibly other OSes

[pombredanne]

• Travis supports Mac slaves.
• Appevyor offers Windows slaves.
• The OpenSuse build service support most major Linux distro and arches.
• CDash (also used for the upstream libarchve CI) seem to offer mac/Gentoo/OpenBSd and win

Getting at least a Mac and Windows would help a lot.

[Changaco]

I do have an AppVeyor account now, so I've added this project to it, now all we need is a PR with the configuration file.

[pombredanne]

the more involved part is about getting a proper build of the libarchive natives for Windows.

[pombredanne]

It works fine with MinGW and mingw-get