improved images upload on the client side

This is the first part of some work regarding the handling of **images** in Discourse. 
This PR concerns only the client side aspect of it:

- [x] images are displayed using a rounded corner to match style (both in topic view and in the composer)
- [x] refactor fileuploader events bindings so they don't bind globally
- [x] check image size on client-side when pasting or droping an image (instead of waiting for the server)
- [x] display an error message when trying to upload several images at once
- [x] changed uploading message position (it was overlapping with the draft status)

#### Rounded corners
![Screenshot_01_04_13_03_00 2](https://f.cloud.github.com/assets/362783/322675/5be2cf5e-9a6a-11e2-9b62-b4db4728c6f5.png)

#### Uploading message's new position
![Screenshot_01_04_13_02_59](https://f.cloud.github.com/assets/362783/322676/5be957fc-9a6a-11e2-8f82-6daeb3be4e36.png)