add Statistics.mean and Statistics.std as aliases for value and uncertainty

[simeonschaub]

My main motivation behind this is that MonteCarloMeasurements.jl already overloads these, so I believe this would make for better interchangeability between both approaches. Personally, I also find them more convenient to write than value and uncertainty, so it also might be worth then exporting them.

[giordano]

This is interesting, but my fear is that one may accidentally strip the uncertainty out in generic code that doesn't know about Measurements, as mean would have a different semantic meaning for, say, Float64 and Measurement. Thus, I'm not really convinced this is a good idea.

[giordano]

Given how a silent conversion to Float64 caused so many troubles in #64 I'm more and more inclined to not do this.