Adding support for animated gifs

I know I'm missing tests.
Also need to update README

Adding the `frame(durationInMs)` filter. 
Its just my OCD but the different comment styles makes me sad:

Single space on either side of `//`:

```
graphicsControlExtensionNode.setAttribute("delayTime", (10).toString) // stupid! "Delay Time (1/100ths of a second)"
```

Double-space before `//`, single-space after:

```
graphicsControlExtensionNode.setAttribute("transparentColorIndex", "0")  // somewhere else has it as 255
```

Double-space on both sides:

```
val byteArray:Array[Byte] = Array(0x01, 0x00, 0x00)  //  The last two bytes...
```