fix a problem when a multi row cell is higher than all the single row cells together

Ignore-this: 42c9682920ebb5295c8dd65fbc0e483

darcs-hash:20140909172542-fae1a-c32a9ee7a4a633ef0855a6f5080a8954dbe3432b
generalize baseline placement for text and inlays and implement sub and sup tags for XHTML output

Ignore-this: 3aa0d5d147decd5f84684c56fda19ba4

darcs-hash:20140909173140-fae1a-64dd7e1e07600b00d0c8642d987b150afc287e8b