Add options to allow use of SSH agent for authentication in addition to
password and public/private key.

Authentication via SSH agent is supported by the underlying ssh2 library
that grunt-sftp uses, and so passing those options to ssh2 via connect()
allows use of that mechanism.
Merge branch 'ssh-agent-support' of git://github.com/kuleszaj/grunt-ssh
tidy
add docs for ssh-agent; update history
Release v0.6.0