Make uppercase letters in hex characters the default, add option to change

Hi @weslly I've been a big fan of your plugin for a while, and I have a new feature to suggest. Depending on the platform (I think), the actual color picker GUI plugin produces either upper or lowercase letters, so you can get different results on different machines. I added logic to `sublimecp.py` to check for `ColorPicker.sublime-settings` and see if the `"color_upper_case"` value is true or false, and depending on that it calls the built-in `.upper()` or `.lower()` function on the `color` variable before it gets sent to the view for replacement.

In addition, I created a sample `ColorPicker.sublime-settings` file with `"color_upper_case"` set to `true`, made a `Main.sublime-menu` file to add the appropriate items to the `Preferences -> Package Settings` menu, and updated the documentation in the README as well as the `gh-pages` branch (that'll be a separate pull request).

I hope you like it, please let me know if you need anything else.

Matt