Return a correct list of available namespaces.

This ensures that inner namespaces take precedence over outer namespaces.

Fixes #40.
XPath support for absolute paths without tests.

This allows Oga to parse and evaluate the XPath expression "/". This expression
can be used to select just the root node/document.

This fixes #35.
Handle lexing of input such as just "</".

Previously this would cause the lexer to go in an infinite loop in the "text"
state machine.

This fixes #37.