appveyor: add configuration file (based on vorbis')

[tmatth]

Testing appveyor webhook.

[vividos]

It seems the AppVeyor builds for this PR fail since the "Visual Studio 2015" image is used, and the image doesn't contain the VS2008 compiler. The build fails at the step where "win32\VS2008\libspeex.sln" is compiled. I guess some new VS2015 project files would be in order to fix this.

[tmatth]

@vividos yeah VS2015 is needed to build ogg (the dependency of speex here).

I will try building without the dependency on ogg so that at least there's something (this will skip buliding the example programs which depend on ogg).

[tmatth]

If someone with access to these toolchains can do one of the following (from best/hardest to worst/easiest):

• create a VS2015 sln file for speex
• build the ogg dependency with VS2015 from a VS2008 image
• add a VS2008 .sln in libogg
• figure out how to do the equivalent of ./configure --disable-binaries for appveyor/win32

that would unblock this.