Update README.md

Modified README.md to describe the purpose of the Simbody3.3 branch, which is to make only changes needed for integration with Gazebo.
Removed "(development)" note in version number for Doxygen.
Merge branch 'master' of https://github.com/j-rivero/simbody into simbody-3.3
Modified MobilizedBody::lock() and lockAt() methods to immediately modify q and/or u in the state to satisfy the requested lock, rather than waiting until the next prescribe() call.

This satisfies Kevin He's request in Simbody issue #20.
3.3 version release
Merge pull request #30 from j-rivero/master

3.3 version release
Added .travis.yml file to simbody-3.3 branch; copied from Chris Dembia's contribution to master branch.
Ask Travis to build with clang as well as gcc.

(cherry picked from commit 1fe03caa08052f76ab5d619da2c7a232b7a3b5a9)
Fix clang warnings.
Fix dependency: liblas -> libblas
Remove libblass dependency
Merge pull request #32 from j-rivero/master

Clean up dependencies
prevent NaN generation when gravity is (0, 0, 0) and we switch from body is excluded to body is not excluded in gravity force appliction.  This is necessary as the rest of the code skips updates when gravity is special cased to zero.
Merge pull request #33 from hsu/simbody-3.3

Prevent NaN generation when gravity is (0, 0, 0) and a previously-excluded body has gravity re-enabled. This needs to be merged into the master branch also.
fix #38: use CMAKE_INSTALL_PREFIX

SimTK_INSTALL_PREFIX was previously used for specifying the
install prefix and CMAKE_INSTALL_PREFIX. This commit changes
to use CMAKE_INSTALL_PREFIX instead.
Issue #38: fix use of SimTK_INSTALL_PREFIX

Fix the use of SimTK_INSTALL_PREFIX for backward compatibility.
Merge branch 'scpeters-issue_38'

Steve Peters sent a pull request for this to the simbody-3.3 branch; this is to copy it to the master branch as well.