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.
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.

(cherry picked from commit dd1b1a9eabbf87fc7a973b53b16da8381a2f1f5e)
Some DecorativeGeometry improvements:
- After discussion with @aymanhab, added to DecorativeGeometry the ability to store and recall a user pointer for use in selection operations for mapping the selected piece of geometry back to the object to which it refers.
- There was already a body id stored with DecorativeGeometry; I added an arbitrary index also that would allow differentiation of several pieces of geometry that might refer to the same object.
- Added missing concrete class overrides for the generic set() methods that return concrete type references for easier chaining.
- Added some missing doxygen.
- Lightly tested.
Merge pull request #35 from sherm1/decorativeGeometryUserRef

Add ability to maintain back reference to user object in DecorativeGeometry. Per discussion with @aymanhab.
Minor fix to SemiExplicitEuler2Integrator -- wasn't properly updating prescribed q's for the double-size step. That could affect error estimates in some unusual circumstances but probably won't have a noticeable effect.
Merge pull request #37 from sherm1/semiExpEuler2bugfix

Minor fix to SemiExplicitEuler2Integrator error estimation
Provide access to the list of prescribed Q and U variables at the System level for integrators or other solvers that may need to know.

(cherry picked from commit 37edc20d1d4e6c9786066e92f99129132f5c8829)