Multiple User Agents and Command-Line Options

I've written most of my intentions in the esantoro_todo.org.

Main points:

- I've made that more best-practices-compliant implementing command-line options handling
- I've implemented custom download directory via command line option
- I've implemented multiple custom user agents
- I've made that username and password are now specified via the -u and -p command line arguments
- i've added some basic documentation (option -h)