Allow for higher quality gifs (higher framerate) and variable playback speed

This adds two new options, `-s` for `speed` and `-r` for `(frame)rate`. I wanted to make GIFs that had a higher framerate than 10 (still the default), since I had a situation where a UI glitch was so fast it was being excluded from the conversion!

This should address #6, although non-integer speeds will (eventually) be rounded to the nearest integer (this is apparently due to the GIF format: http://superuser.com/questions/569924/why-is-the-gif-i-created-so-slow#comment696679_569967).

One idea I had was to instead expose the raw GIF delay option, but figured that might be too complicated for this tool.

Examples:

### 60fps
`$ gifify -r 60 -n mm3-jump.gif`
![60 1](https://f.cloud.github.com/assets/90831/1836253/5e01a448-7400-11e3-9d28-5563aee52ae7.gif)

### 30fps
`$ gifify -r 30 -n mm3-jump.gif`
![30 1](https://f.cloud.github.com/assets/90831/1836255/6808fa9a-7400-11e3-969b-df492f6764fd.gif)

### 30fps @ 2x speed
`$ gifify -r 30 -s 2 -n mm3-jump.gif`
![30 2](https://f.cloud.github.com/assets/90831/1836256/70707096-7400-11e3-8411-e6c50f472b03.gif)

### 10fps (default)
`$ gifify -n mm3-jump.gif`
![10 1](https://f.cloud.github.com/assets/90831/1836258/80044500-7400-11e3-800f-f1aea3290c95.gif)

### 10fps @ 2x speed
`$ gifify -s 2 -n mm3-jump.gif`
![10 2](https://f.cloud.github.com/assets/90831/1836259/86454b30-7400-11e3-8587-1875a39f5145.gif)

### 10fps @ 3x speed
`$ gifify -s 3 -n mm3-jump.gif`
![10 3](https://f.cloud.github.com/assets/90831/1836260/8cca7a3e-7400-11e3-9212-c39125aed2fb.gif)

### 10fps @ 6x speed (!)
`$ gifify -s 6 -n mm3-jump.gif`
![10 6](https://f.cloud.github.com/assets/90831/1836261/95b88230-7400-11e3-9cec-5e7d1d78eaff.gif)