Use new syntax of url built-in template tag.

The syntax changed to wrap the main argument in quotes (to be consistent
with other Django template tags it seems). This is optional in Django 1.3
and 1.4, but mandatory in the recently released Django 1.5.

After this commit you must be running Django 1.3 or later.

Closes #52.
getLaTeXScientificNotation() now handles negative numbers correctly.

Previously it would fail for negative numbers due to the need to take the
logarithm of the value to get the exponent.