Use X and Y, not two Y's, for rigid's sigma

This was a bona-fide bug that we caught with our bigger tests (not yet
merged).
Move cmake modules up a directory

We don't need the Modules/ subdirectory, so let's axe it.
Use git-describe for the version

Since we now have a good tagging/versioning strategy, let's use
git-describe to produce our software version. This has the added bonus
of producing libraries with more exact version names, which is better
for avoiding weird confusions while we develop.

We don't automate SOVERSION, since this should be more of a manual
upgrade step.
Add version.cpp file

This forces the project to recompile with each new commit, and provides
a character array for the library/callers to use for debug output.

Taken from
http://stackoverflow.com/questions/1435953/how-can-i-pass-git-sha1-to-compiler-as-definition-using-cmake.
Add cmake status message with version string
Merge branch 'autoversion' into release/v0.1.1
Add v0.1.1 changes in preparation for release
Merge branch 'release/v0.1.1' into production