Update build lifecycle docs

Updates the documentation to reflect the current lifecycle for builds. Also tries to offer examples for what types of commands to run during the various hooks. This is my best guess based on my interpretation of the code, but I'm open to being wrong about any of this. Here's the new copy:

1. Clone project repository from GitHub
2. `cd` to clone directory
3. Checkout commit for this build
4. Run `before_install` commands
   * Use this to prepare the system to install prerequisites or dependencies
   * e.g. `sudo apt-get update`
5. Run `install` commands
   * Use this to install any prerequisites or dependencies necessary to run your build
6. Run `before_script` commands
   * Use this to prepare your build for testing
   * e.g. copy database configurations, environment variables, etc.
7. Run test `script` commands
   * Default is specific to project language
   * All commands must exit with code 0 on success. Anything else is considered failure.
8. Run `after_success` or `after_failure` commands
9. Run `after_script` commands

And here's a screenshot of what it looks like:
![screen shot 2013-09-02 at 3 53 04 pm](https://f.cloud.github.com/assets/1420926/1069311/28abd9ce-1412-11e3-8381-43a11d0b6526.png)

Feel free to let me know what you think!

Ref #239