Replace pageX and pageY with clientX and clientY to fix scroll issue

[rylax]

Fixing issue #11 which caused a bug that whenever a user scrolls the color picker position got messed up.

The reason for this issue was pageX and pageY being used instead of clientX and clientY. You can read more about it here: https://stackoverflow.com/questions/6073505/what-is-the-difference-between-screenx-y-clientx-y-and-pagex-y

[deanmarcussen]

Any update on this, looks like it will fix the issue?