sftp command on linux and readme

While figuring out this [issue](https://github.com/gc3-uzh-ch/elasticluster/issues/104), I had thought the root cause was that the sftp command was not working. It wasn't but I did figure out what was wrong with sftp. On Linux, sftp does not have the ```-i``` option and so that had to be changed. It now tests if running on either Mac or Linux.

I also changed a bit of the Readme to simplify the install from source section. Ansible is already specified in setup.py and so installing it first isn't needed, nor is cloning the git repo needed either. pip can install straight from github.