Updated based on Ember 1.3.1 bind-attr helper

NOTE: This version drops support for `unit` keys. It does, however, automatically add `px` to numeric values in the same way that jQuery does. It also changes the preferred name to `bind-style`, rather than `bindStyle`.
Updated to match the new implementation
Minor tweaks
Fixed case of title
Updated to be more accurate
Fixing erroneous camel-casing
Minor: No longer adding empty 'style' attrs.

If there are no styles (i.e. every bound style evaluated to null, we don't need to output a style attr.
Minor: Better handling of empty string values.
Expanded helper documentation.