Replace the paramiko implementation with direct Popen calls for ssh

Paramiko is very strict about authentication, and we cannot figure out what configuration settings we must set in order to have ssh calls work with hosts that have properly configured keys. So we are replacing Paramiko with bare ssh calls from Popen.

this commit fixes #16 
This might be unrelated to this commit, but do you think we should add "nohup" down in the RemoteShellClient class (as a side effect of async=True)?