Use ruby tags and styled spans for furigana instead of CSS tables.

Make use of native ruby tag support in WebView for SDK 11+, but
fall back on manually aligned spans on earlier versions of Android.
These look better and don't overlap other elements like the
previous CSS table solution does.
Fixes Issue 1648.
Merge pull request #23 from hssm/Issue1648

Use ruby tags and styled spans for furigana instead of CSS tables.