Rewrote an attribute spec.

This way the spec doesn't depend on the attributes object being a Hash.
Use XML::Attribute for element attributes.

Instead of using a raw Hash Oga now uses the XML::Attribute class for storing
information about element attributes.

Attributes are stored as an Array of XML::Attribute instances. This allows the
attributes to be more easily modified. If they were stored as a Hash you'd not
only have to update the attributes themselves but also the Hash that contains
them.

While using an Array has a slight runtime cost in most cases the amount of
attributes is small enough that this doesn't really pose a problem. If webscale
performance is desired at some point in the future Oga could most likely cache
the lookup of an attribute. This however is something for the future.
Support for querying attributes using XPath.
Lex XPath axes without a node test.