Make chat input box auto-resize

issue #765 :
replace the chat text input with an auto-resize textarea which can be expanded or shrunk depending on content:
- I have used the "oninput" event (propertychange for IE) to take into account copy-paste and Undo actions .
- repalced keyup with keydown so we can handle the submit action(pressing Enter) before adding a newline to the textarea.
- scrollTop is used to determine the overall height of textarea (instead of ScrollHeight) for cross-browser consistency.
- I've modified submitChat function to avoid submitting an empty text.