cmake: enabling examples to be installed; fixed CTest paths

with `-Dpfasst_INSTALL_EXAMPLES` the executables of the example programs
can be installed into $PREFIX/bin -- enabled by default

also fixed the paths of the CTest test cases; somehow this fixes a
couple of failing tests for me on my system o.O

fixes #99
cmake: don't install examples by default

also: fix Travis
Merge pull request #101 from torbjoernk/feature/cmake-tweaks

LGTM