added command line options, project reorganisation

Hi Jonathan,

I found this project particularly interesting for automating taking screenshots. I added a few more options and  made a reorganisation of the code. 

The idea is to have one instance of ui-screen-shooter and include in the Xcode project only the config files. With the previous approach if you use ui-screen-shooter in a large number of projects each update of ui-screen-shooter means update it in 20 projects.

I was trying the script with a project that took a while to compile, and it was a pain to have compile it every time I wanted to change the config-automation.js, --force and --skip-build are very useful for this.

When using iPhone5 in simulator list it said that there was a name conflict with iPhone5s. By adding trailing " " on the call it gets solved and does not affect other simulators.

The script creates a temporary symbolic link to .lib/ where the capture.js is kept. It's useful to get rid of the complete path in the import on config-automation.js and it helps to easily make the script work independently of the path. The only problem I see is that if the script fails in the middle the ./lib link remains.

Also I updated the Readme with the new features and changes.

Feel free to add the changes.
Thank you for the script.
Regards, Juan.