InlineFlow should always inherit text-width from base style

Fixes #222.
auto merge of #1349 : dhedlund/servo/issue_222, r=jdm,me

Fixes #222.

Implements 'text-align' inheritance for inline elements.  The text alignment code has been changed a couple of times since the ticket was originally opened and was improved to inherit the text-align property from from an ancestor element.  This addressed the issue, so long as no inline elements had a text-align property defined.  Both Firefox and Chromium ignore any text-align properties attached to inline elements, but none of the specs are explicit about ignoring the property or inheriting only from non-inline elements:
http://www.w3.org/TR/CSS2/visuren.html#inline-formatting
http://www.w3.org/TR/CSS2/text.html#propdef-text-align

This is my first stab at writing any code that touches the DOM node tree.  Based on a few observations, I made the assumption that the `self.base.node` of a InlineFlow always corresponds to a BlockFlow-based node, no matter how deep inline DOM elements are nested; there was no obvious way to traverse Flow-traited objects directly and I'm not sure if it's possible to get from a FlowData struct back to a non-child {Block,Inline}Flow (probably intentionally).

I could've kept traversing the node tree directly, checking against each node style to ensure it didn't have an inline display property, but I could not create a scenario where the `base.node` was ever an inline.  It also feels like a code smell to be walking up the tree at all, especially for stylesheet properties.  Feels like it should eventually be handled in style::properties directly as being conditional inherited?