Merge pull request #47 from heuermh/master

minor doc fix, CVS --> git
fixing a problem caused by update to Sifts file format
adding a test case to make sure the SiftsXml parser can parse the a
Sifts file correctly
New getChains() method.
A convenience constructor that takes a URL.
Wrote a version of getGroupsWithinShell that takes an Atom and a
blacklist of ResidueNumbers.
Wrote a method getGroupDistancesWithinShell.
update to demo how to access Structure objects
updating the demo how to load mmcif files