Added bookmark formatting (colors, bold/italic) to addBookmark

This commit adds formatting to bookmarks, allowing the user to set the color (as a red, green, blue tuple) and bold/italic, in PDF readers that support this feature (including Adobe Acrobat).

![bookmark_colors](https://cloud.githubusercontent.com/assets/3883947/3578291/7094862c-0ba7-11e4-9005-d9fac4e49290.png)

The screenshot above was taken from Adobe Acrobat running on Linux, created using the following code:

```
page = 0
outputpdf.addBookmark('Regular', page)
outputpdf.addBookmark('Bold', page, bold=True)
outputpdf.addBookmark('Italic', page, italic=True)
outputpdf.addBookmark('Bold italic', page, bold=True, italic=True)

outputpdf.addBookmark('Red', page, color=(1.0, 0.0, 0.0))
outputpdf.addBookmark('Orange', page, color=(1.0, 0.5, 0.0))
outputpdf.addBookmark('Yellow', page, color=(1.0, 1.0, 0.0))
outputpdf.addBookmark('Green', page, color=(0.0, 1.0, 0.0))
outputpdf.addBookmark('Blue', page, color=(0.0, 0.0, 1.0))
outputpdf.addBookmark('Indigo', page, color=(0.25, 0.0, 0.5))
outputpdf.addBookmark('Violet', page, color=(0.5, 0.0, 1.0))

outputpdf.addBookmark('Fit whole page', page, fit='Fit')
outputpdf.addBookmark('Fit page horizontal', page, fit='FitH')

outputpdf.addBookmark('Bold red', page, color=(1.0, 0.0, 0.0), bold=True)
```
In addition, the page fit setting can be modified using the `fit` argument. This allows the user to specify a whole page fit rather than the previous default of a horizontal page fit.