to_rational_coeffs: return None if all numerical coeffs are Rationals

    Do not return something if numeric coefficients are already
    Rationals or else a recursion error at Line 974 of polyroots will
    ensue.

    Formerly, the presence of a sqrt(non-number) was giving a return
    value which lead to the recursion error under certain conditions.
add note to nsolve and watch for spurious 'method' keyword
edit docstring; remove obsolete comment; add coverage tests
ellipse: handle prec=None case and arbitrary ellipse failure
_tsolve: return None for all unhandled expressions
unrad: remove unused "all" keyword (use minpoly instead)
6393,6451,8617: watch for RootOf.evalf interval collapse and non-sym gen

At low precision, the interval may collapse before a
successful starting point is found for the secant method.
When the interval collapses, that value is returned for
root.

During evaluation, replace non-symbol generators with a symbol
so evalf doesn't have problems.
_eval_interval checks for zoo

This fixes the integral integrate(x*sin(3/x), (x, 0, 1)) as reported at
http://stackoverflow.com/q/27517233/161801.

The result of the added test was verified by integrating the derivative of
Si(1/x) from x = 0 to 1 in WolframAlpha.
Merge pull request #8629 from asmeurer/eval_interval_zoo

_eval_interval checks for zoo
Implement Abs._eval_is_even/odd helpers

fixes #8631
Revert 893daf4 and add test for issue 8642

fixes #8642
Merge pull request #8612 from smichr/misc

misc issues related to solve and RootOf
Merge pull request #8638 from skirpichev/8631-fix

Implement Abs._eval_is_even/odd helpers
Merge pull request #8644 from skirpichev/8642-fix

Revert 893daf4 and add test for issue 8642