Fix scientific formatting

[krlmlr]

• Largely uses the same code as decimal formatting
• Negative and NA/Inf are displayed properly (but is the alignment correct)?

Build output: https://travis-ci.org/hadley/colformat/builds/248730482#L632.

Fixes #8.

Next steps:

• Simultaneously compute data for both decimal and scientific notation
• Get rid of the ugly format(format()) pattern

[hadley]

We should probably change the default NA etc styling so it's more readable on travis (and in general in consoles with black background and fewer colours)