implement Canvas#initialize spec
initialize canvas with instant variable grid
implement Canvas#render
Implement Canvas#display
raise error upon initialization if canvas is empty
add spec to check #initialize raises invalid dimensions error
refactor specs
add #draw_line 'draws a horizontal line' spec
add #draw_line 'draws a vertical line' spec
fix typo in horizontal line spec
add specs for raising errors in #draw_line, fix punctuation on error messages
add helper bracketing methods for Canvas class
raise errors when bracketing off the board
fix coordinates in spec testing for orthogonality
implement error-testing in #draw_line
implement #draw_line
implement #draw_rect specs
implement #draw_rect
implement multiple drawings spec
add #fill specs