Switched to using CMake's internal mechanism for handling library header
files by

- Providing a macro stp_add_public_header() that the object libraries
  used to build libstp can use to set their PUBLIC_HEADER property.
- libstp's PUBLIC_HEADER property is set to concatentation of the
  PUBLIC_HEADER property for its constituent object libraries.
Build system now creates STPConfig.cmake file in build and in install
directory which allows external projects that use CMake to easily
import STP as a target.
Added simple example project that demonstrates some of STP's C API
and how to use STP as an external CMake project.
Use CMake's library path (set in src root) for output path for libstp.