Improvements to avatar default, user scrolling, add subtitle support

First of all, thanks for this great starting point for any messaging application. My use case for this control is for group chat so most of my changes make my use case possible. This PR does a few things:

* The **avatar delegate and datasource methods are now optional** and if they are not implemented, it is assumed that you don't want them.
* A new `JSMessagesViewAvatarPolicy` has been added: `JSMessagesViewAvatarPolicyOutgoingOnly`. This should be self explanatory.
* When the avatar related methods are implemented, `avatarImageForIncomingMessageAtIndexPath:` should now be used in place of `avatarImageForIncomingMessage`, etc. **This allows for different avatars depending on the message**.
* **A new property** was added to `JSMessagesViewController`: `preventScrollToBottomWhileUserScrolling`. When this is set to `YES`, the tableview won't automatically scroll to the bottom when scrollToBottom is called if the user is scrolling the view. This is very useful in busy group chat cases so that **users can place their thumb on the screen to read that their own pace instead of constantly being jumped to the bottom**.
* In conjunction with the point above, a new public method has been added `scrollToRowAtIndexPath:atScrollPosition:animated:`. This method respects the property above so that you can continue to program scrolls and still allow the user to read at his/her own pace.
* You can now place **a subtitle below the bubbles**. This is useful for showing someone's name in a group chat. It is optional per cell via `hasSubtitleForRowAtIndexPath:` and `subtitleForRowAtIndexPath:`. You can see a screenshot below.
* The documentation has been updated to reflect these changes.

I have also taken the liberty of bumping the `podspec` version to `2.1.0` and tagging the repo (I'm not sure if the tag merges though).

I hope you guys find these changes as useful as I do.

![screenshot 2013 08 13 00 44 35 copy](https://f.cloud.github.com/assets/261668/952621/25882c78-03d4-11e3-9f4a-24e67c719cdc.png)