Create `Makefile` to build UDT4 and C++ examples.

Created a `Makefile` that will download and build the UDT4 C++ reference
implementation and build it along with our C++ example programs. We're
going to use the server side of the C++ programs as part of our strategy
to ensure that we conform to the protocol, testing against the UDT4 C++
reference on the platforms where it will easily compile, ensuring
protocol parity between the C++ and JavaScript implementations.
Build on Linux.

Needed to include `unistd.h` in `sdk/integer/server.cpp` for the sake of
the `sleep` function.
Build on Travis CI.

Invoke `make` before install at Travis CI.

Fixed an "unexpected operator" complaint from `bash` when building on
Travis CI.

Closes #29.
Merge branch `makefile` into `master`.