Make custom form checkbox and radio buttons a little more customizable

Added the following variables:

+$custom-form-border-size:               1px !default;
+$custom-form-input-size:                16px !default;
+$custom-form-check-size:                14px !default;
+$custom-form-radio-size:                8px !default;
+$custom-form-checkbox-radius:           0px !default;

For check mark, use U+2A2F instead of U+00D7 (see http://en.wikipedia.org/wiki/X_mark), U+2A2F has a better typography baseline for generic alignment.

Also clean up and the _variable.scss a little and fixed some minor issue on the renaming of $form-label-xxxx variables (update them to match the names in _forms.scss)