806: hyper.rewrite(Sum)

Change return value to Piecewise
Add tests
denest powers in powsimp so 4**s*2**(x-2*s) -> 2**x

Should this only be done if the base is a Number?
3341: allow factors of coeff to enter powers

* i order to make this work, multiplicity was made to handle Rationals.
Merge pull request #2031 from smichr/3341

3341 - allow powsimp to denest and process coefficients
added elementary arithmetic operations on algebraic numbers
explain odd test
hadamard transpose test is blind to ordering
replace string MatMul._entry test
remove commutative assumption on MatrixElement
Transpose guards against None result
hadamard product is not commutative
Euler totient function as symbolic SymPy class
Merge pull request #1975 from mrocklin/matrix-cookbook-1

More Matrix Expressions
fixed bug in ``minpoly_pow``; added ``minpoly_neg``
Merge pull request #2033 from v9joshi/3773_acos

Issue #3773: Error in acos(x).rewrite(atan)
Add Varun Joshi to the AUTHORS/aboutus. Welcome to SymPy!
MatrixExprs use real/integer/complex assumptions
move test_closed_group from sets to __init__
add Q.real/integer/complex_elements predicates
apart and cancel are nc-aware