Only split draw expression on single colons

The expression passed to `TTree.Draw()` is split on colons to change the
axis ordering from ROOT's `y:x` to the more conventional `x:y`.  However
this mangles expressions containing the C++ scope operator `::`, such as
`var_A:TMath::Exp(var_B)`.  This commit modifies this behaviour to
ignore multiple adjacent colons, and split only on single colons.
Merge pull request #615 from stevensons/colons-in-draw-expr

Only split draw expression on single colons