Download and use libcurl from WinRPM on Windows

This fixes #9, please verify that it doesn't cause any problems on Linux or Mac.

I had to tweak the `@c` macro slightly to allow for a variable `libcurl` - the dll from WinRPM has a version suffix. There might be a better way to accomplish the same thing, I'm not sure. The version suffix is a good thing in the end because the Git bundled with Windows Julia comes with its own 32-bit libcurl.dll, which won't work for this package with 64 bit Julia.