broken with two <md-content> columns

[garretwilson]

This seems to work if I have a single <md-content layout="column"> column, but if I have a <md-content layout="row"> containing two <md-content layout="column">, each containing a table, the fixed table headers don't work --- the fixed header and the table are showed side by side in each column.

(Note that I'm creating two columns with two tables side-by-side to try to simulate a fixed column of row headers; see #27. If that issue is addressed, maybe I wouldn't even need two <md-content layout="column"> anymore.)