Move to use token, instead of api

Since indico api is outdated, .

I migrated it to indico token. However I'm not sure if other auxiliary functionalities was affected. Since I only needed the main functionality, and they're working. I thought it could be useful, but let me know otherwise.

Here are some features:

• the input argument --API_TOKEN(unused in master), takes the string generated at /tokens/ webpage: (available only when a token is generated)
  • No secret key is needed for token.
• I couldn't open the iCalendar when I'm using token. This would need additional work so I didn't progress much on it..
• I also start to use acron instead of acrontab, I added something in the readme.