multialignment is used when '\n' (newlines) exist in text

If there are *no* newlines, `text._multialignment == None`

If `text._multialignment` has *not* been set, matplotlib will
default this value to equal `text._horizontalalignment`,
for reference.
Merge pull request #26 from mpld3/text-multialignment

multialignment is used when '\n' (newlines) exist in text