Support to render icon with text label

Add three parameters at icon symbolizer. This parameters are the
label text, horizantal align and vertical align. The text will be
handled in the renderer to add a text label to image icon.
See https://github.com/openlayers/ol3/issues/886.

The body of the commit message can be several paragraphs, and
please do proper word-wrap and keep columns shorter than about
74 characters or so. That way "git log" will show things
nicely even when it's indented.

Further paragraphs come after blank lines.