added text-align to css

This adds css text-align property to the css string.

It follows the same pattern for the other css properties (i.e. uses the first color/font/etc).

This is only tested on the newer photoshop file format, as I do not have access to an older version of photoshop to create the test.

The test is run on a test file called alignment_modes.psd that has a a layer for each of the available CSS alignment modes (left, right, center, justify.)  Any of photoshops justify modes map to the same css justify mode.