Fix GitHub issue #78.

References to multiplyColour should've been as a method of this sniff,
not a global function.
isStringEmpty changes: closes GitHub issue #76.

1. Removed inexplicable duplication of isStringEmpty, util version kept.
   SC 1.1.1 changed to use this.
2. If isStringEmpty is not passed a string, it will be treated as empty.
Unwind remaining WCAG2 dependencies in the Section 508 code.

Closes Github issue #66.