Polynomial remainder sequences, gcd, and resultant

Documentation on the computation of gcd's and resultant's of
polynomials via remainder sequences, with emphasis on subresultants.
Computing resultant with subresultants

Reordered the loop in the computation of subresultants to include
one more scalar subresultant, namely, the resultant. Another loop
in the resultant function can then be omitted.

Notes:

1. The subresultants and the resultant of the pair `(f,g)` as computed
   are actually those of the pair `(g,f)` if `\deg(f) < \deg(g)`.
   This may have effect on the signs, but that is harmless in general
   when the resultant is only used for eliminating a variable.
   I have not changed this to save some time in the computation.
   Correct signs may be restored afterwards if that is necessary.

2. The resultant of `(f,0)` is zero indicating that `f` and `0` have
   common roots, namely every root of `f`. However, if `f` is a
   non-zero constant, it has no roots. In this case the resultant
   should be different from zero -- in fact one, by convention.
   I have not changed this as it would break the test in
   test_euclidtools.py.
integrals: An amendment to spde()

The iterative version of spde() currently assumes that the
coefficient  a  of  Dp  were invariant in the loop, in which
case the values of the multiplier  alpha  are powers of  a.
Otherwise the offset  beta  of the solution will be incorrect.

I have added two test cases where  a  is not invariant.
In the general case,  alpha  can be used instead of a power of  a
to construct the correct value of  beta.
Don't force push to sympy_doc
Deploy the docs as part of the script phase in Travis

This has several advantages:

- If it fails, the build will fail.
- We do not need to rebuild the docs. They are already built from the test
that builds the docs.
- We do not arbitrarily only deploy when the first test in the matrix is
finished. The deploying is now tied to the docs building matrix.
Fix bash syntax in .travis.yml
Fix name of deploy script in .travis.yml
Generalize physics.quantum.OuterProduct so that it can handle scale kets/bras and sums of kets and bras
Unit tests for updated OuterProduct in the physics.quantum module
Evaluate dif for inequalities only if arguments are reals

fixes #8288
product should return 1 if term=1, fixes #8301
Fix tests
Sum evalf of non rational term
doit() keeps unevaluated Product, if _eval_product returned nan
Eval Abs(exp(x*I)) as 1 if x is real

- also rewrite re and im in terms of each other
Improve handing of Abs(b**e) when b is real
fixes Abs(x)**-3 bug
Integrate functions multiplied by Heavisides

- uses manualintegrate to deal with complicated cases
Merge pull request #8214 from asmeurer/travis-docs

Travis docs
Add _eval_Eq methods for Interval, ProductSet and FiniteSet