Add support for creating grids from string form

The constructor form

    Grid("""
    ...
    ###
    """)

will now create grids.

"\n" indicates the end of a row, "." indicates a space, and any other character indicates the presence of a wall.

-
This fixes issue #6
rename state/col variables to 'cell' for clarity
Merge pull request #7 from NotBobTheBuilder/multiline-string-maze

Multiline string maze