Add login command
Send error messages to stderr and return non zero value

Refactor out code
Use py3-compatible function to write messages to stderr

Ignore scrapy.cfg auth
Fix prefix in environment variable that contains SH's key
Fix hardcoded strings
Add key validation

Add a 'fatal' function to print a message to stderr and exit with error code 1
Refactor 'is_valid_key' function
Replace re.findall() by re.match()
Replace fatal function by a click-based counterpart
Merge pull request #4 from bertinatto/login_command

Add login command