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

update subplot.jl cat -> concat

concat(..) -> Gadfly.Aesthetics.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