Experimental CMake support

The files in this directory are mostly for dealing with dependencies.

We use ExternalProject to download and build missing dependencies, during CMake's configuration time.

Still TODO for CMake support:

• Benchmarks.
• Top-level examples other than helloworld.
• Leaf examples (e.g. trace/examples/ dir).
• Exporters other than stdout.
• CI (Travis) support.
• No shared library for now.

Quickstart:

cmake -H. -B.build
cmake --build .build
(cd .build && ctest --output-on-failure)
./.build/examples/helloworld/opencensus_examples_helloworld

Using .build as the build dir so that it doesn't collide with the uppercase BUILD file on Windows.