[why]
The importing application may have some specific security requrirements that necessitate
a change to the http.Client or http.Transport used when fetching the xml from the UPnp server.
For example, the importing application may want to restrict localhost calls which could be
made by an attack server on the local network.
[how]
Create a global HTTPClient which defaults to http.DefaultClient. This allows the importing
application to modify this global if it wishes to make changes to the http.Client/http.Transport
used when fetching the xml from the UPnP server.
