Update sphere.py - calculation of the gzz gravity gradient component

I modified the gz function for the purpose of calculate the gzz gravity gradient component.
Update sphere.py - corrected gzz function

I corrected a line indentation problem in the gzz function.
Removed logger module. Cleaned all packages and respective recipes except gravmag
Trying out the Travis CI system
Travis doesnt like tabs
Some tabs were left over
Travis build error using pip. Moved things to apt-get
Travis had import errors. Trying system site packages in virtualenv
Not using make test in Travis and not building before pip install
Fixed typo in nosetests command
Travis unit test error due to nose looking in src dir for cython modules
Removed setup.cfg and set nosetests flags in Makefile instead
Added build status to readme
Merge pull request #29 from leouieda/travis

Added support for Travis CI building and testing
Removed old .hg files
Merge branch 'master' into remove-logger
Removed the imaging compare recipe because it was broken
Finished removing logger from gravmag and recipes
Merge pull request #30 from leouieda/remove-logger

Remove logger module
Removed deprecated run-changed script