getElementByTagName problem with namespaced elements

Hi,

Using getElementByTagName by selecting elements prefixed with namespaces ("stream:features", "stream:error") don't work in several browsers like Chrome or Safari (but it works in Firefox)

Replacing getElementByTagName calls with getElementByTagNameNS in those lines where you want to find prefixed namespaced elements solves the issue and you don't have to first check `getElementsByTagName("stream:features")` and then `getElementsByTagName("features")`

If you accept this patch, [strophejs-bower](https://github.com/strophe/strophejs-bower) repository should be updated too :-)
Cheers