added collision example in response to question.  resolves #617.
hopefully fixes (very) low probability failure on unit test
```Matlab

                           < M A T L A B (R) >
                 Copyright 1984-2014 The MathWorks, Inc.
                   R2014a (8.3.0.532) 64-bit (glnxa64)
                            February 11, 2014

To get started, type one of these: helpwin, helpdesk, or demo.
For product information, visit www.mathworks.com.

To reproduce this test use rng(2042137770,'twister')

[Warning: Disabling autosave for simulink blocks (to avoid generating a lot
of
*.mdl.autosave files in your directory.  If you aren't a regular Simulink
user
and don't want this disabled, comment out this section in addpath_drake.
To disable this warning in the future, add
  warning('off','Drake:DisablingSimulinkAutosave')
to your matlab startup.m]
[> In addpath_drake at 74
 In addpath_drake at 4]
gradients generated by user and taylorvar don't match for output 3
Error using valuecheck (line 78)
Values don't match.
   Index       Value       Desired Value
  -------     --------    ---------------
    (1,6)  9399.488358     9399.488358
    (3,6)  9399.961561     9399.961561
    (1,8)  9399.488358     9399.488358
    (3,8)  9399.961561     9399.961561

Error in geval (line 79)
           valuecheck(fdf{j},varargout{j},options.tol);

Error in testAngularvel2rpydotMatrix>testGradient (line 14)
   [~, ~, ~] = geval(1, @angularvel2rpydotMatrix, rpy, option);

Error in testAngularvel2rpydotMatrix (line 3)
testGradient();

Error in fevalPackageSafe (line 21)
 feval(f);

<test_name>util/test/testAngularvel2rpydotMatrix</test_name>
<error_id>Drake:ValueCheck</error_id> <error_message>Values don't match.
   Index       Value       Desired Value
  -------     --------    ---------------
    (1,6)  9399.488358     9399.488358
    (3,6)  9399.961561     9399.961561
    (1,8)  9399.488358     9399.488358
    (3,8)  9399.961561     9399.961561
</error_message>

```
moved most urdf-specific parsing into addRobotFromURDF (instead of RBM or RB).  started working on sdf parser.
randn -> rand to avoid (very) low probability unit test failure with huge values.
adds support for parsing models w GAZEBO_MODEL_PATH
initial pass at converting to objs
able to parse quite a bit of the drc_practice_task_2.world .
successfully loads the terrain task from drcsim.  (except for the ground plane, which it throws a warning for).
brought over a single sdf world + the minimal set of models so that I'd at least have a unit test on the sdf support in the main drake.
fixed typo caught by build server
code review from hongkai
Merge pull request #632 from RussTedrake/sdf

provides very minimal SDF support