Add new statistics function column_counts

[roed314]

This is a function on a statistics table that is used to determine the values taken by a column and the counts of rows with each value.

sage: db.ec_nfcurves.stats.column_counts('field_label')
{u'2.0.10691.1': 1L,
 u'2.0.11.1': 56587L,
 u'2.0.127285811.1': 1L,
 u'2.0.1879.1': 1L,
...

sage: db.ec_nfcurves.stats.column_counts('field_label',constraint={'number':1})
{u'2.0.10691.1': 1L,
 u'2.0.11.1': 30614L,
 u'2.0.127285811.1': 1L,
 u'2.0.1879.1': 1L,
...

[coveralls]

Coverage decreased (-0.03%) to 60.512% when pulling 34fa7b4 on roed314:counts into f3bc9ee on LMFDB:master.

[JohnCremona]

Testing now