First iteration on a simple fetch_tle cmd
Simplify delete Satellite option
Add fetch_range_tle command
Merge pull request #57 from satnogs/fetch_tle

Management command to fetch TLEs. Two commands:
* Fetch certain Satellites: ./manage.py fetch_tle 07530 23439
* Fetch a range of Satellites: ./manage.py fetch_range_tle 20000:21000