Merge pull request #1774 from asmeurer/travis_test

Rename travis_test.sh to test_travis.sh
Merge pull request #1816 from skirpichev/3626

Fix for #3626

Function._eval_series() does wrong prediction for number of taylor
terms if the argument of function is non-atomic.
Merge pull request #1822 from mrocklin/log-product

log(Product(...)) -> Sum(log(...))
Added more intro content in the mechanics docs.
Added more intro content in the mechanics docs.
Responded to pull request comments.
Addressed the additional pull request comments.
An error in solve for Piecewise.
rsolve: use limit() (with fallback to subs()) to substitute initial values of n in the generic solution of the recurrence equation
Merge branch 'master' into 3669-rsolve_hyper
Merge pull request #1798 from prasoon2211/series-expand

Fixed 'maximum recursion depth reached' issue for addition of Order term to another expression containing an order term.
rsolve: simplify handling of the initial conditions
rsolve: tune exception handling for limit(), fallback to subs() in case NotImplementedError
Merge pull request #1839 from skirpichev/3669-rsolve_hyper

Fix #3669: rsolve_hyper can't find solution for F(n) + F(n - 2) + F(n - 1)==0
Merge branch 'master' into 3553-DeprecationWarning-on-compile_rule-doctest
Merge branch 'master' into 3377-Remove-as_coeff_terms-and-as_coeff_factors
Remove debugging leftover
Fix imports in hyperexpand.py
Rename IndexPair to Hyper_Function and subclass Expr

This class actually represents a hypergeometric function.
This makes it explicit.
s/x > 0/x.is_positive/ and similar changes

bool(x > 0) doesn't return a meaningful result when x is
symbolic, whereas x.is_positive does.