scale_colour_manual

Added a manual scales file ```scale_colour_manual.py```

It'll take a manual list of colours and apply them in place of the default colors found in ggplot/components/colors.py.

It doesn't yet do everything scale_colour_manual does in ggplot2, but it does discrete variable colors. 

example:

```python
from ggplot import *

color_list = ['#FFAAAA', '#ff5b00', '#c760ff', '#f43605', '#00FF00', '#0000FF', '#4c9085']
lng = pd.melt(meat, ['date'])

print ggplot(lng, aes('date', 'value', color='variable')) + \
    geom_point() + \
    scale_colour_manual(values=color_list) + \
    ggtitle('With manual colors')
```