Checks if XDA_DATA_HOME in os.environ before use

Checks if XDA_DATA_HOME in os.environ before use

Additionally when XDA_DATA_HOME doesn't exist the cookie jar location
defaults to os.environ["HOME"] + '.local/share/uzbl/cookies.txt'
however when these strings are concatenated they are missing the path
seperator '/'. Problem solved by using the builtin os.path.join to join
the environ path and jar location.