Closes #159 by making the cmake_package feature complete.

The cmake_package by default sets CMAKE_INSTALL_RPATH, and
CMAKE_INSTALL_RPATH_USE_LINK_PATH by default. It also uses the global
debug flag to determine the value for DCMAKE_BUILD_TYPE.

Also for CMake builds we need to setup CMAKE_PREFIX_PATH with the
directories of our dependencies, this allows CMake projects calls
to find_* to first look at hashdist build versions first.
Remove all redundant values from cmake packages.
Add a test to verify the new cmake package works.
Hand-merge .travis.yaml

Also noting (courtesy Robert Maynard)

> I didn't clearly state this in the commit message but by explicitly
adding a CMake variable to the extra section it will override any of the
values that cmake_package sets. This happens because CMake treats the
last occurrence of a value is the preferred one, and I make sure to add
the extra section last.
Merge pull request #183 from robertmaynard/cmake_package

Cmake package