Variable/document name changes for syntax highlighting.

Changed the grammar used for syntax highlighting: the PLSyntaxHighlighter apples syntax coloring (rather than highlighting). As we use "highlight" to apply to several other features (line number, animating brackets, etc.), this change clears up the usage in this case. The name PLSyntaxHighlighter remains, but all documentation now describes it as coloring the text.
Fixed documentation for colorTextStorage: method in PLSyntaxHighlighter.

The documentation for this method still referred to an old method for syntax coloring.
Removed two unused instance variables from PLEditorViewController (previousEntry and editedRange).

Removed the textStorageWillProcessEditing: and PLEditorViewController no longer registers for the NSTextStorageWillProcessEditing notification as these were used to set the editedRange variable.