Added support for some kinds of text alignment-baseline.

This patch provides a partial fix for the missing formatting
of the "alignment-baseline" attribute for the "text" SVG
tag.  It adds most of the alignment baselines, although it
omits the language and text-specific ones; indeed, only
Western text will be aligned. The "middle" baseline is
treated as "central" due to the lack of access to font
x-height information in Cairo.
Added non-standard "display-anchor" tag to align text vertically.

The "display-anchor" attribute of the "text" tag is an
invention of the patch author. It provides the ability to
align text (as opposed to just a font) vertically at either
the "top", "middle", or "bottom" of the text. If
display-anchor and alignment-baseline are both provided,
display-anchor takes precedence. The display-anchor
attribute is only processed for Western fonts.
Merge branch 'text-vertical-alignment' of git://github.com/BartMassey/CairoSVG into BartMassey-text-vertical-alignment
Style