initial beta release for Holberton School review

This release incorporates all of the major functions of a working shell.

Features:
Custom builtins:
- help
- cd
- setenv
- unsetenv
- env
- alias
- unalias
- history
- arsine
Checks PATH for executable files like /bin/ls if no directory is given
Builds a custom abstract syntax tree to take in and execute more complex commands with pipes, redirection, etc