Get CI for windows builds working

[RadicalZephyr]

There are two broad routes here:

• use AppVeyor
• use Travis' new support for windows via Git Bash

For the AppVeyor route, building with MinGW is problematic because of a bug in rustc (I think?) that causes a linker error.

undefined reference to `_imp____acrt_iob_func'

This is apparently caused by a mismatch between mingw versions.

Discussions and possible workarounds I've found so far:

nabijaczleweli/cargo-update#74
rust-lang/rust#47048
racer-rust/racer#919 (comment)
rust-lang/rust#47048 (comment)
https://github.com/nabijaczleweli/cargo-update/blob/master/appveyor.yml#L16-L17

Example appveyor.yml