Implemented custom format strings.

Format strings can contain the fields `{method}`, `{uri}`, `{status}`, and `{response_time}`.
Loggers can be created using
```
Logger::new(from_str(<format string>))
```
to use a format string, or
```
Logger::new(None)
```
to use the old default format. Calling `from_str` on an invalid format string returns a `None` (as specified in the `FromStr` trait), and therefore invokes the default format.
Can you make the default color softer, as in Morgan, so it doesn't look like everything else on the screen (i.e. like Rust's debugging statements)?