Fixes #8368

```
>>> laplace_transform(cosh(t), t, s, noconds = True)
s/(s**2 - 1)
>>> laplace_transform(sinh(t), t, s, noconds = True)
1/(s**2 - 1)
```

I got my hands on a copy of Prudnikov's book. The weak inequalities here (m+n>=p and s+t>=u) definitely violate his conditions as printed. However, this is the absolute bare minimum needed to get a condition to fire for sinh and cosh.

In fact, none of the conditions as written in Prudnikov are satisfied for the particular products that sinh and cosh produce. I'm not sure whether that means he has a typo or his result simply doesn't apply to these cases.

If adherence to Prudnikov is preferred, we still end up fixing the s+t>0 typo in condition 30.

Added tests to test_laplace_transforms.

Forgot the import statement.
Integral objects now convert Abs instances to Piecewise.
Fixes #8430.
```
>>> integrate(Abs(x),(x,0,1))
1/2
>>> integrate(Abs(x),(x,-1,1))
1
>>> integrate(Abs(Abs(x)),(x,0,1))
1/2
>>> integrate(exp(Abs(x-1.0/2)),(x,0,1))
-0.351278729299872 + 0.606530659712633*E
>>> integrate(Abs(x-1.0/2)+x,(x,0,1))
0.750000000000000
```

The parent class of Integral (AddWithLimits) will now convert
instances of the Abs class to Piecewise objects. To do this I added a
function rewrite_abs to the file containing the Abs class
(complexes.py). Additionally, I made the private function
_eval_rewrite_as_Piecewise assume arguments are real unless they
explicitly say they aren't, which is more lax than the previous
version.
Minor whitespace typo.

Undid changes for issue 8368. Basically I'm bad at git.

More undo-ing. See previous commit.

One more undo.
Rewrote rewrite function using already existing utilities.
Added test_issue_8430 to test_integrals.py.
Adjusted test_Abs_rewrite in test_complexes.py
to reflect changes made to _eval_rewrite_as_piecewise.