Add methods to alter scan threads in scan_template.rb

Add the ability to adjust the scan threads for a scan template through the gem. These two methods correspond to the two options in the scan template editor:
![Scan threads in template editor](https://cloud.githubusercontent.com/assets/8505926/4449779/ca1d63a4-481e-11e4-8123-2ec5f8289b4e.png)