Fix conversion of Complex to Quaternion/Octonion

[dkarrasch]

Not sure if this addresses #29, but at least it allows the Base promotion to work. This method was previously not tested, and probably not working.

[goretkin]

Can you speak to the semantics of this operation? If I got it, this operation is consistent with the definition at https://github.com/JuliaGeometry/Quaternions.jl/blob/master/src/Quaternion.jl#L15 to convert Complex to Quaternion.

One possible answer to the semantics question is that this multiplication corresponds to rotation in a specific plane. Have there been cases where you've wanted to do this?

[dkarrasch]

The changed methods currently don't work at all, because there is no Quaternions.imag(::Complex). Remember that imag is not imported from Base, and I think this is done on purpose. I noticed this while writing tests for LinearMaps.jl, but didn't need it in the end, so we can as well close this.

[dkarrasch]

Gentle bump. I still needed it in LinearMaps.jl tests, in which I needed a non-commutative number type multiplied with complex numbers. I renamed the PR to reflect that this actually fixes broken conversion methods.

[dkarrasch]

I guess this is superseded by #52, but I'll leave it open in case there is something controversial about #52.

[sethaxen]

Closing this now because #52 was merged.