Add a default colour to the frontend job types

Hey Mike,

Congrats on popping this into the repo :+1: 

We've had a couple of jobs listed on the WordPress community site we're using this on and I noticed a minor issue that I should've picked up on when I did https://github.com/mikejolley/WP-Job-Manager/pull/3 for you.

There should be a default color on front end for the custom job types. 

Before:
![before](https://f.cloud.github.com/assets/1377956/793835/794ac846-ec43-11e2-9202-98af140c9a2b.png)

After:
![after](https://f.cloud.github.com/assets/1377956/793836/7f3badf6-ec43-11e2-8b46-895ff06338f6.png)

Sorry for not picking that up in the original pull request!

Cheers :smiley: