Support handle of images from ftp.
ml() can built url to these images, either fetch didn't accept them.
Update phpdocs of checkFileStatus
Added media_isexternal()
tests for media_isexternal
apply media_isexternal
Merge remote-tracking branch 'origin/master' into fetchftp
apply media_isexternal in ml()
Merge pull request #225 from splitbrain/fetchftp

Add media_isexternal