`fm logs <sitename>` should show frappe dev server logs and other fixes

[Xieyt]

This PR:

• Solves: fm logs <sitename> should show frappe dev server logs #26
• Add -f short flag for --follow in fm logs <sitename>
• Fix port checking implementation, now using python package psutil for Linux and lsof command for osx #77.
• Minor status message changes and code cleanup here and there.
• Provides service name hints in help for --service flag in fm logs and fm shell subcommand.