SemanticMediaWiki throws ImportError if BeautifulSoup is not present, and BeautifulSoup warning is removed

geopy gives a warning to all its users if the optional dependency BeautifulSoup is not installed. It would be better if only users who touched functionality needing BeautifulSoup were affected; and, that users who attempt to use functionality needing BeautifulSoup see an `ImportError` on initialization, rather than a warning on initialization followed by an undefined variable exception. 

This pull request defines BeautifulSoup as either the package or, if not available, `None`, and if the consuming `SemanticMediaWiki` geocoder is used with a BeautifulSoup of `None`, an `ImportError` is thrown.