BUG: Display of AssetUpload upload fields

* Replaced file upload icon
* Fixed padding
* Replaced hard coded width of droparea with min-width

Tested on Firefox, Chrome, IE8, and IE9

### Before
#### Assets
![assets-before](https://f.cloud.github.com/assets/984753/563582/ec531cfe-c4ec-11e2-83da-f0215a79020f.png)

#### Insert Media
![insert-before](https://f.cloud.github.com/assets/984753/563583/f5a6e7ae-c4ec-11e2-9060-e9d36fa6722b.png)


### After
#### Assets
![assets-after](https://f.cloud.github.com/assets/984753/563584/08a2ae06-c4ed-11e2-9876-2215dc46db76.png)

#### Insert Media
![insert-after](https://f.cloud.github.com/assets/984753/563585/1502157e-c4ed-11e2-8240-e84c67569e87.png)