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
Merge pull request #2357 from asmeurer/pprint

Fix typo in pprint docstring
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.
Merge pull request #2345 from toolforger/deprecation-warnings

Eliminated DeprecationWarning from test_pickling.
Fix a white space error

This was introduced in e2a415797d4adb81551c5331b1adfdb4a6ecbe94.
Added removed test
Added slow test for mechanics.

Added a test which generates the equations of motion for a bicycle, linearizes
it, and compares the linear equations to benchmark values (numerically, using
eigenvalues). Most of the code was already in an example in the documentation,
just with the slow parts commented out - now it is actually a test and uses the
slow decorator.