Import not imported exception HeuristicGCDFailed
HeuristicGCDFailed was being raised without importing it in polys/heuristicgcd.py.
Change absolute import to relative import
log expands negative numbers (just like Numbers)

The prinipal branch value is returned when the argument is negative:

log(-3) -> log(3) + I*pi  (previous behavior)
log(-sqrt(3)) -> log(sqrt(3)) + I*pi

Symbolic expressions which are negative are not rewritten.
[skip ci] fix test
replace respects commutativity
Add Nathan Woods to AUTHORS/aboutus. Welcome to SymPy
Put a space. Change 'anormal' to 'a normal'.
Merge pull request #7752 from smichr/nc-replace

replace respects commutativity
Merge pull request #7750 from smichr/log-neg-arg

log expands negative numbers (just like Numbers)
Merge pull request #7756 from debugger22/plane-typo

Put a space. Change 'anormal' to 'a normal'.
Merge pull request #7628 from debugger22/undefined-HeuristicGCDFailed

Import unimported exception HeuristicGCDFailed
More code quality improvements
Merge branch 'master' of https://github.com/sympy/sympy into dyadicvector