Extract common logger functionality into Racker::LogSupport

Builds on #3:
- Extracts common logger behavior into Racker::LogSupport module.
- Renames logger references from `log` to `logger`.
- Adds tests for Racker::LogSupport.
- Fixes an issue where bad log levels would report that they had been set correctly.

```ruby
# Example of bad log levels incorrectly reporting success.
# Based roughly on current Racker::CLI
options[:log_level] = :emergency

# This call will fail to find a related Log4r log level and will default to Log4r::INFO
log.level = get_log4r_level(options[:log_level]) 

# But still reports that setting the log level to :emergency was a success
log.info("Log level set to: #{options[:log_level]}")
```