shell: Add pushd/popd functions

Signed-off-by: Esben Haabendal <esben@haabendal.dk>
shell: Add unittests

Signed-off-by: Esben Haabendal <esben@haabendal.dk>
Integrate nosetests in Travis CI setup

Signed-off-by: Esben Haabendal <esben@haabendal.dk>