fixed v0.4 error with function cat(..) by changing to concat

update subplot.jl cat -> concat

concat(..) -> Gadfly.Aesthetics.concat(..)
Merge pull request #494 from kbuzby/master

fixed v0.4 error with function cat(..) by changing to concat
changed syntax to match v0.4 and added Compat dependency for v0.3 compatability

moved using Compat from misc.jl and ticks.jl to Gadfly.jl for consistency