install header files to deployment directory

A header file is any file under src/ which matches the glob *.h.
Relative directory layout is preserved. Fixes #9.
CMake: add pkg-config file

Add a generated pkg-config .pc file to the files installed by the
install target. Fixes #8.
CMake: add SOVERSION and VERSION property to shared library

Ensure that the installed files correspond to the usual Linux naming
convention.
CMake: set version in one place

Add version setting to the top-level CMakeLists file and use it for
setting pkg-config, VERSION and SOVERSION properties.
Merge pull request #13 from csrf/headers-and-pkg-config

General tidyups to build system