Be compatible with c++11 standard.

* Modified the source code so that gcc 4.8.1 and clang 3.2 would compile the source code with `-std=c++11` enabled. While the changes seem minor, please review them to make sure I didn't mess anything up.
* Modify travis testing to build both without the c++11 flag and WITH the flag. So now, travis is building 4 times. This required installing gcc-4.8 in travis. (EDIT: see below).
* Travis testing now uses the '-j8' flag for `make` and `ctest`, though I am not positive it is having an effect. (EDIT: yes it is having an effect).
* Create CMake flag to optionally use the c++11 flag (`SIMBODY_STANDARD_11`). Right now, it defaults to OFF. This is because Gazebo would rather not use c++11 for now, and while OpenSim requires the c++11 flag, OpenSim trunk isn't currently used by too many people. Eventually, the default setting should change to ON.

## Testing
The following setups all pass with and without `-std=c++11`:
* Ubuntu 13.10, gcc 4.8.1
* Ubuntu 13.10, clang 3.2
* OS X 10.9, Clang 5.0.0
* Travis (Ubuntu 12.10), gcc 4.8

Other setups:
* Travis (Ubuntu 12.10), clang 3.3 -- ONLY without `-std=c++11`
* Windows, VS2013 (c++11 flag not relevant) tests pass

The only setup that does not pass is travis (Ubuntu 12.10) clang 3.3 -std=c++11. I thought maybe it was because travis was using a different version of clang than I was using locally. But that is NOT the issue. Locally, I tried using clang 3.3, and all tests passed. Therefore, I am disabling the travis clang -std=c++11 build for now. I do not know why this build isn't passing tests (two tests), and I don't really have a way of exploring why the tests are failing.

The only thing left to do is:
- [ ] Finish getting travis to NOT build with `-std=c++11` and clang.