[ADD] QWeb: xml namespaces support in dynamic template

This is very similar and almost copy pasted from the implementation in
static template, as the main method `compile_node` will either call
`_compile_static_node` (which we patched in the static implementation)
or `_compile_directives` which will call `_compile_tag` in the cases
where xmlns matter (at least that's my understanding for today). So we
patch `_compile_tag` the same way than `_compile_static_node`. It would
be nicer if the method wasn't completely different and in two parts, but
hey.

This method is not always called on leaves, so setting options['nsmap']
to a copy before processing the content of the node is necessary (a node
with a nsmap and a t-attf- having a child with an nsmap: the child has
to be aware of the nsmap of its parent).

Note that we set the xmlns attribtue before calling
`_compile_all_attribute`, which will probably break xmlns set by t-*
tag. However, we don't plan to support that the moment.