Make linkcheck work on servers without HEAD, and add FTP status checking

[iamed2]

curl -I uses HEAD requests, which don't work on all servers. Links really only need GET, so we can fallback to that if HEAD fails.

I also added support for FTP status checking, once I found a way to get that information out. I needed to suppress the regular output of curl, which required redirecting to a file through curl -o (so I needed to use /dev/null instead of Julia's devnull).

This ended up making the regex matching code simpler.

[mortenpi]

It seems that curl on Travis is too old for this, hence the errors. It should be 7.35.0 in Ubuntu 14.04, however, from the curl man page on --write-out:

scheme         The URL scheme (sometimes called protocol) that was effectively used (Added in 7.52.0)

Is there a way to avoid using %{scheme}?

[iamed2]

Is there a way to avoid using %{scheme}?

Likely; I'll figure something out

[mortenpi]

Sorry for the delay, LGTM. Thanks! Will aim to backport and tag this shortly.