Fixed error in tests in mechanics.

Some tests in mechanics (or rather, various versions of one example problem)
had an error in the definition of the _kinematics_ of the rolling disc.
However, if you did the derivation by hand and also made this error (the
improper application of the 2-point theorem for acceleration for the rolling
w/o slip case) you would also get this error. That is the reason why the
original hand-calculated values matched the SymPy output. All is fixed now.

Additionally, the kinematic differential equations are calculated, rather than
being hard-coded. This should be more robust and less likely to be a source of
confusion/error in the future.
Also added eigenvalue test to mechanics.

In the rolling disc example test file, now the eigenvalues are checked against
benchmark values. For all parameters equal to unity and an upright disc, the
linearized equations have a critical point (all eigenvalues are 0) at 1 /
sqrt(3). This is now checked.
Fixed test in mechanics.

The test was written to check eigenvalues, which return a dictionary. These
eigenvalues are (all) supposed to be 0. The issue was that the 0 in the test
wasn't comparing to a SymPy 0 - fixed by comparing to S(0).
Fixed mech test failing?

One of the corrections to the tests seemed to fail on all the Python versions
except 3.2, which happens to be what I'm developing on. I've changed the
assertion from an equality test to a subtraction/expand/0-equality assertion -
I think this should make it more robust. I also tested it w/ python2.7,
successfully.
Merge pull request #2339 from gilbertgede/mech_test_fix

Fixed error in tests in mechanics.