Add name aliasing for boolean operations

[bartvanerp]

In ReactiveBayes/ReactiveMP.jl#170 we added boolean operations to Bernoulli random variables. For user convenience, these operations should also be able to be performed using "regular" mathematical expressions, i.e.:

# current implementation
z ~ AND(x, y)

# improved user interface
z ~ x && y
z = x && y

[bvdmitri]

Fixed with #18