Delegate background_color to the master_cell [closes #412]

I went ahead and implemented @jonsgreen's suggested fix. Works great and closes #412 

Here is my modified version of @ottost's provided example

```ruby
data = [[
{colspan: 3, content: 'Iniciativa', align: :center, background_color: 'AAAAAA'},
{colspan: 4, content: 'Alcance', align: :center, background_color: 'AAAAAA'},
{colspan: 8, content: 'Estrategia', align: :center, background_color: 'AAAAAA'},
{colspan: 4, content: 'Enfoque', align: :center, background_color: 'AAAAAA'},
{colspan: 2, content: 'WOW', align: :center, size: 10, background_color: 'AAAAAA'}
]]

# data = ["foo " * 40] * 31
pdf = Prawn::Document.new
pdf.table(data, row_colors: ['DDDDDD','EFEFEF'])
```

Result:

![Screen Shot 2013-03-08 at 2 39 43 PM](https://f.cloud.github.com/assets/40662/238669/1662c05a-8841-11e2-8c53-48d62a3b927e.png)