Limit scroll to rects to inside content view.
Limit scroll to rects to inside content view.

Fixes #33

programming086/DTRichTextEditor,liulinjian/DTRichTextEditor,fishermenlabs/DTRichTextEditor,liulinjian/DTRichTextEditor,programming086/DTRichTextEditor,Cocoanetics/DTRichTextEditor,fishermenlabs/DTRichTextEditor