Add headers and precompiled libraries for libcurl support on all platforms

This turned out to be a much more involved task than anticipated, but now I've learned the process for adding static libraries to the build process on each platform. I expect most other libraries we'll need will not require adding files for each platform, but as far as I could find it is impossible to share header files for libcurl across different platforms due to some architecture-specific checks that libcurl performs at compile time. Keeping precompiled static libraries is also undesirable in general, but the build process for libcurl on Android requires the downloading the Android NDK, building a standalone compilation toolchain, then cross-compiling for each Android architecture needed (see the 'Android' section here: http://curl.haxx.se/docs/install.html). However, users wanting to build Android will need the NDK regardless so theoretically we could also build libcurl along with our code. Incidentally, I'm open to finding other networking support libraries :)