Add `highlight-symbol-occur'

This function runs `occur' for the symbol at point.
Updated change log.
Merge pull request #3.
Clean up doc strings. Thanks jturner314.
Extract functions for highlighting and unhighlighting.
Use highlight-symbol-remove-symbol to remove all.
Replace hi-lock usage with font-lock.
Fix typos. Thanks jturner314.
Allow specifying fonts instead of colors.
Add highlight-symbol-list-all.
Updated change log.
Replace hi-lock dependency.
Update copyright year.
List compatibility with Emacs 24.