fix xaxis/yaxis rendering, add keyword axes in PlotLayout

[hhaensel]

Although we do no longer develop our own implementation of Plots in favour of PlotlyBase, I feel that the current implementation of axes is confusing. (See #66 )

Currently, axis setting of the rendered PlotLayout is set by the xy field of PlotLayoutAxis and not by the keyword/field xaxis/yaxis.

I propose to change the rendering method such that we to chose the axis type according to the axis field so that

julia> render(PlotLayout(yaxis = [PlotLayoutAxis()]))
Dict{Symbol, Any} with 1 entry:
  :yaxis => Dict{Symbol, Any}()

For those who would want to keep the possibility to determine the axis type by the xy field, I have added a new keyword axes that keeps the behavior of former xaxis/yaxis.

julia> render(PlotLayout(axes = [PlotLayoutAxis(), PlotLayoutAxis(xy = "y")]))
Dict{Symbol, Any} with 2 entries:
  :xaxis => Dict{Symbol, Any}()
  :yaxis => Dict{Symbol, Any}()