prefix support, needs more testing

I think it is impossible for a 1 letter prefix to cause ambiguity with
this implementation, but I'm not sure.
don't remove_prefix from base_units

kg was breaking down to k*g, which is why base units shouldn't have
prefixes
improve test
better prefix implementation

need to consolidate units after remove_prefix I think
defined hash and isequal so UnitSymbol works as dict key

also exported prefixes and removed testPrefix.jl from test.jl since I
rewrote who prefixs work, and plan to test them in testQuantitys.jl
added prefix tests
export things and add more nice symbols
add back in show methods for UnitSymbol and Prefix
Merge branch 'alt_prefix'