Function to simplify units, e.g. m/ft -> dimensionless

@ajkeller34, unless I'm mistaken, the following functionality (`simplify`) does not yet exist. Happy to try and implement it if it doesn't:

``` Julia
notsosimple = 1u"m/ft*kg"
simple = simplify(notsosimple)
@assert units(simple) === units(1u"kg")
```

It should be possible to determine which unit to use of m or ft, the same way it is done for +, via`promote_type`

Start by examining the existing arithmetic behavior that uses promote_type, then trace where a simplify entry point would fit. The issue's example defines the expected result: simplifying 1u"m/ft*kg" should leave the compatible mass unit and produce units equivalent to 1u"kg"; add coverage for that behavior.

Written by the indexing model from the issue text.