Added some implementation to Document along with some unit tests.
Document now supports basic cursor movement with the arrow keys,
pressing enter, and saving.

Also moved and slightly updated .gitignore to reflect new executable.
Hacky fix to make Model compile
Added more sophisticated cursor movement, so that it knows where
the line ends, but will only jump to the end of a shorter line if
it has to.
Added the ability to poll a Document for information about its
contents and its cursor location, as well as the ability for a
Document to read from the filesystem when it is initialized.
Merge pull request #9 from Fourth-Doug-Coders/add-doc-impl-with-tdd

Added Implementation for Document, along with unit tests to ensure correctness
- looks good