Option for colorization and some fixes

Hi,

Two changes in which you might be interested:

1. Function `colourise` will not colourise the output if `getOptions("testthat.use_colors")` is `FALSE`. This addresses issue #153 (which I filed myself....). I'm not sure if there was any other way to turn that off.

2. Some fixes in the docs of `R/test-that.r`. It was triggering warnings in `R CMD check` about `\em` command and non-ASCII characters.
Hmm, I'd probably skip this to and use just `isTRUE(use_colors)`