[UPDATE] add opacity functionality, and set half opacity when drawing
[FIX] drawing opacity value
[UPDATE] add draw line functionality and update version to v0.0.4
[UPDATE] README.md
[REFACTOR] take out jQuery dependency & bug fixes, update version to v0.0.5
[UPDATE] README.md
[FIX] README.md
[UPDATE] README.md
[REFACTOR] mouse events & _internal object