Fixed links.

Fixed quotation mark on pcc.pdf.

Cleaned up all sheets; inline code styles, other corrections. Made al…

…l-in-one sheet.

Fixed missing braces in dictionaries sheet.

Added cheat sheet for Plotly.

Added Django cheat sheet.

Added matplotlib cheat sheet.

Added cheat sheet for Pygame.

Added cheat sheets for classes, files and exceptions, and testing.

Added cheat sheets for if and while, and functions.