FTP URLS not resolving correctly

[gothub]

The function isResolvable() does not resolve FTP URLs, as the urllib2.urlopen() library function that is called does open the url but returns a status of "None".

The long term solution is to refactor metadig engine so that it can use python 3 (NCEAS/metadig-engine#211).

The short term solution is to parse the url, detect that it is an FTP URL and use the urllib2 FTPHandler or another library that handles FTP URLs properly.