Indexing rework for 0.2

[brainandforce]

Because we currently disallow bare integer indices for AbstractCliffordNumber instances, we could rework the indexing scheme to allow for integer indices if the inputs to getindex are passed through to BitIndex:

getindex(x::AbstractCliffordNumber, i::Integer...) = getindex(x, BitIndex(x, i...))

This is a relatively naive definition; we may want to perform checking so that a BitIndex of a grade constrained to be zero by the type of x automatically returns zero without performing any construction.

[brainandforce]

This actually may not be a breaking change, and can go into the 0.1 series.