Previously we only supported HTTP_PROXY because the proxy support in Go
was lacking in older versions. Now that there is
`golang.org/x/net/http/httpproxy`, we can rely on that instead to gain
consistent support for HTTPS_PROXY and NO_PROXY.

Done by `dep init`

After the last howeyc/gopass update, hitting Enter after password input
makes the next printed line appear indented to the position of the
password prompt. I'm not sure what causes this, but as a workaround,
skip the howeyc/gopass dependency altogether and interface directly with
crypto/ssh/terminal.

Fixes #1262

See https://groups.google.com/forum/#!topic/golang-nuts/kTVAbtee9UA