libwebsockets

additional casts allow test server build as cpp

With these explicit casts that are not needed in C, it's possible to build
the test server using g++ like this, after building and installing the
library.

g++ -DINSTALL_DATADIR=\"/usr/share\" -ocpptest test.cpp -lwebsockets

Add a small documentation to README.coding

Signed-off-by: Andy Green <andy.green@linaro.org>