Add a new shell_out that works with AST-commands

[aaronbog]

This pull request contains two main parts.

1. the commandAST folder

• Nodes contain all of the data structures used to create our AST
• Visitors are full of example visitors
• abstractTypes file exists to counter circular dependency issues in Python
• command.py contains a piece of code called command that will convert string-based commands over to the AST variant
• test.py shows the command.py functioning
• visitor.py contains syntactic sugar to make the visitors more usable. There is also a set of use case examples.

2. an implementation of the shell called ast_shell_out.py

• this is a backwards compatible shell_out function.
• there do seem to be some small bugs left regarding the return time and return codes of ssh commands.
• at the bottom of the file you can see the three ways you can provide commands to this function.