Set log level with environment variable, fixes #19

Default log level now set to warn.
Other log levels can be set by setting the BUNYAN_LOG_LEVEL environment variable.
For example:
`BUNYAN_LOG_LEVEL=info node .`
or
`BUNYAN_LOG_LEVEL=error npm test`
Merge pull request #25 from Fodi69/master

Set log level with environment variable, fixes #19