Add install target

Add an install target to CMake. Also add a compatibility install target to the top-level Makefile but it's a bit ugly. Installation location is set via the standard ``CMAKE_INSTALL_PREFIX`` variable.

Since it's trivial, add ``make install`` as a step which must succeed in Travis too.

I would not cry if only d63ad0d were cherry-picked but the remaining commits help maintain the use of the top-level Makefile as the "start of all things".

Closes #89.