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.
Eliminated DeprecationWarning from test_pickling.
Fix issue 3744: simplify nseries output with factor_terms()

    The calculate_series should always return a non-zero
    value for nontrivial expression.  Let's do some cheap
    simplifications to check the result is not zero and
    call nseries() with increased n if it is.
Drop workaround for issue 3744 in limit()
Add._eval_as_leading_term(): check for zero output

    _eval_as_leading_term should always return a non-zero value
    from nontrivial expression.  Lets substract the returned
    expression (equal to zero) from the original one an restart
    _eval_as_leading_term() calculation.  See also issue 3744.
Fix failing test in test_integrals.py
Fix typo
Fix typo in pprint docstring
Solving variable coefficient PDE's
Also made slight changes in documentation
Replaced * with spaces
Used __name__ instead of class_key
Merge pull request #2357 from asmeurer/pprint

Fix typo in pprint docstring
Made the following changes
1] Corrected two minor bugs in pdsolve
2] Extended 1st_linear_varcoeff_pde for the following subcases
   a] d = 0, and one of b and c is 0
   b] b = 0
   c] c = 0
   in the PDE b*u.diff(x) + c*u.diff(y) + d = e
Update run-solvers.py

Fixed a syntax error in line 27
Merge pull request #2360 from twobitlogic/master

Update run-solvers.py
Merge pull request #2352 from shikil/docfix

Fix typo
Merge pull request #2339 from gilbertgede/mech_test_fix

Fixed error in tests in mechanics.