Improved Travis CI integration.

 - Install samtools - needed for bams. Going with version 0.1.19 - newish but not too new.
 - More of a framework for installation of dependencies - provide scripts with some place on the PATH to put things (namely ${BUILD_BIN_DIR} which gets injected into the script runtime).
 - Run install script with -x so the actual commands get printed, should help with debugging.
 - Prettier reporting of commands being run by planemo at runtime.
 - More output information at repo init time - tell developer where install script is and how to register with Travis CI.