resolves #179.  mex rigid body constraints now use the updated drake mex...

... pointer logic, which locks the mex file until all objects are deleted.
We do not check if `subclass_name` is actually derived from `DrakeMexPointer`. Can we add the following code after line 92?
```
bool subclass_check = isa(subclass_name,"DrakeMexPointer");
if(!subclass_check)
{
  mexErrMsgIdAndTxt("Drake:createDrakeMexPointer:InvalidSubclass","subclass_name is not a valid subclass of DrakeMexPointer");
}
```