Merge table metadata properly for astropy.vizier.vizquery

astropy.vizier.vizquery returns Table instances whose columns have
(unicode) string metadata for UCDs and floating point metadata (which is
sometimes NaN). This patch adds support for concatenating
(astropy.table.vstack) tables returned by astropy.vizier.vizquery.

Here is an example:

    import astropy.table
    import astroquery.vizier

    astropy.table.vstack([
        astroquery.vizier.vizquery({
            '-source': 'USNO-B1.0',
            '-c': astropy.table.Table([
                astropy.table.Column([100], '_RAJ2000'),
                astropy.table.Column([20], '_DEJ2000')
            ]),
            '-c.rs': '10'
        }),
        astroquery.vizier.vizquery({
            '-source': 'USNO-B1.0',
            '-c': astropy.table.Table([
                astropy.table.Column([90], '_RAJ2000'),
                astropy.table.Column([20], '_DEJ2000')
            ]),
            '-c.rs': '10'
        })
    ])