Make Sorin's prompt more friendly with solarized colours

Since solarized uses the *bright* version of colour as some variation of the background colour, the bright versions of yellow and green doesn't exist, looking like this:

![sorin-solarized](https://f.cloud.github.com/assets/275754/101469/4a2e14c2-68cd-11e2-8a23-0ed93b1baf2a.png)

Without using brighter colours looks like this:

![sorin-solarized-friendly](https://f.cloud.github.com/assets/275754/101471/5a3240c8-68cd-11e2-86df-4eb5b761f5ca.png)

Since the solarized colourscheme is quite popular I thought it could be helpful.