Add table @rowheader support for PDF output #1462

[Gist](https://gist.github.com/emth/6448121). Something like this would've worked as well:

    <xsl:template match="*[contains(@class, ' topic/tbody ')]/*[contains(@class, ' topic/row ')]/*[contains(@class, ' topic/entry ')][1][ancestor::*[contains(@class, ' topic/table ')][1]/@rowheader = 'firstcol']" priority="1">
        <fo:table-cell xsl:use-attribute-sets="tbody.row.entry__firstcol">
            <xsl:apply-templates select="." mode="processTableEntry"/>
        </fo:table-cell>
    </xsl:template>

    <xsl:template match="*[contains(@class, ' topic/tbody ')]/*[contains(@class, ' topic/row ')]/*[contains(@class, ' topic/entry ')]">
        <fo:table-cell xsl:use-attribute-sets="tbody.row.entry">
            <xsl:apply-templates select="." mode="processTableEntry"/>
        </fo:table-cell>
    </xsl:template>

But I wasn't sure what the policy regarding the `@priority` attribute is, so I ended up doing it the same way as the `<simpletable>` `@keycol` attribute is handled in `tables.xsl`.