Fix cd behavior with absolute paths and home directory

[mvz]

Summary

Correctly handle ~ and absolute paths in aruba.current_directory

Details

• Limit the API of ArubaPath to just a few methods, so its main function is pushing and popping directories.
• When converting ArubaPath to string, special case directories that are absolute paths or start with a ~.
• Add specs for this behavior
• Add scenario for cd-ing to the home directory

Motivation and Context

Fixes #418.

How Has This Been Tested?

Specs and a cucumber scenario were added.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (cleanup of codebase without changing any existing functionality)
• Update documentation

Checklist:

• I've added tests for my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[mvz]

Merging this since the build errors are unrelated.

[olleolleolle]

This is super!