On-board AppVeyor CI automation

In order to validate that ongoing changes are compatible with Windows, continuous integration testing should be added to validate the correctness of each and every change.

There is currently an appveyor.yml file checked in, but, at least in my testing, does not result in a passing AppVeyor build.

I've kicked off a one-off CI run with the current appveyor.yml file: https://ci.appveyor.com/project/AndrewGaspar/rust-bindgen/build/1.0.9

The current issues I've found:

• GNU build
  • fail when attempting to run ./ci/test.sh, which does not seem to exist in this repo
• MSVC build
  • rustfmt not installed
  • LLVM 3.9 Test Failures (after fixing rustfmt)
    • header_issue_769_bad_instantiation_test_hpp (different output)
    • header_objc_template_h (different output)
    • header_type_alias_template_specialized_hpp (panic)
    • test_header_contents (panic)
    • test_multiple_header_calls_in_builder (panic)
  • LLVM 4.0 Test Failures (after fixing rustfmt)
    • test_header_contents (panic)
    • test_multiple_header_calls_in_builder (panic)

I've been testing locally with MSVC + LLVM 4.0 + x64 and have been unable to repro the test failures. My local configuration is with Windows 10 1703, LLVM 4.0.1, latest rust stable and nightly toolchains, and the VS 2017 MSVC toolset.

Start with appveyor.yml and the linked AppVeyor build, then inspect the missing ci/test.sh reference and the listed GNU/MSVC test failures. Update the CI configuration so GNU and MSVC builds run successfully, including rustfmt and LLVM 3.9/4.0 cases, and verify the named tests no longer fail.