changes to support Amazon EC2 instances via addprocs

In order to provide an ability for users to use ```addprocs()``` on EC2 instances from a local Julia console, the following changes have been done.

1. ```addprocs()``` enhanced to accept a keyword argument ```sshoptions```. The requirement came up in supporting access to EC2 instances which would usually require the specification of a keyfile using ```ssh```s ```-i``` option and also  ```-o``` options to disable the prompt for adding a new verified host.

For example, sshoptions may be specified by the user as ```"-i /home/amitm/ec2instkey.pem -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no"``` 

2. ```start_remote_workers()``` changed to use the remote hostname from the machinename supplied. It currently uses the ip-address provided by the remote host as part of the initial handshake. This does not work for EC2 instances  since they are behind a firewall and NATed. The EC2 instance has a public ip-address(and hostname) different from the private ip-address actually bound to the NIC on the remote host. 

The function has been changed to use the hostname from the machinename supplied.