Add ability to change log level

[RachitSharma2001]

Addresses #932

Adds the -log-level flag. From my analysis, it seems like there really are only three kinds of logs made by the gcs server:

1. Info logs: Http request logs and some other information logs made by main.go
2. Warning logs
3. Error logs

Nonetheless, the user can specify, with a string, one of the 7 accepted levels from the logrus package: fatal, panic, error, warning, info, debug, or trace, and the appropriate log level will be set.

[fsouza]

Thanks for contributing!