Sbuercklin/simple array rules

[SBuercklin]

Addresses #12 by implementing rrules for A * s, s * A, A / b where A is an AbstractArray, s is a scalar, and at least one eltype(A) or eltype(s) is a Unitful.Quantity.

This was a little tricky because we had to define two different rrules so as to not overwrite the default methods for CommutativeMulNumber in ChainRules.jl. The definitions here guarantee that:

1. We have Quantity{T,D,U} where T <: Union{Real,Complex} for all Quantitys involved, basically enforcing the CommutativeMulNumber requirement on the backing value
2. We have at least one Quantity involved, either in the AbstractArray or the scalar. This means we do not invalidate ChainRules.jl. This lets us e.g. multiply an array of Quantitys by a scalar