Fixes weird "NaNs" in mpl converted plots.

Well, this is on top of #1411, so the diff is really ugly.
It take like an hour to find the issue, but the solution seems simple...
We need to pass the mpl text object properly but avoiding the title and axes names, because in Bokeh they are outside the plot region wheres in mpl is inside of it... so the NaN where is fact the places where mpl believe the title and axes has to be...
The fixes is just keep a register of the title and axis names and avoid to be converted by the draw_text method. 

Thanks.