* Add .travis.yml configuration file for Travis CI
This builds Linux x64 clang 6.0-dev debug/optimised variants.
* Make clang/libc++ locations configurable from cake command-line.
Added new cake command-line args for Linux/Darwin builds:
--clang-executable=PATH
--clang-install-prefix=PATH
* Refactored config.cake to remove duplication between Linux/Darwin build variants.
* Added `build-clang.sh` and `build-libcxx.sh` helper scripts.
* Add Linux build status badge to README.
* Make Appveyor badge link to build status page.
* Ignore Linux CI failures for clang optimised builds.
They are currently failing due to a compiler bug.
