Apache logging

**The goal of this changeset is to reused the well known Apache log formatting syntax to
define the Go-Json-Rest access log.**

First, compatibility with the existing JSON logging is maintained. This JSON logging feature may be deprecated in the future is favor of a more powerful one. But this is not the purpose of this changeset.

With this new feature, the user can define his own record format:
```go
rest.ResourceHandler{
        LoggerFormat: "%t %r %s %b",
}
```
or pick a predefined one:
```go
rest.ResourceHandler{
        LoggerFormat: rest.CommonLogFormat,
}
```
or just just ignore this field and get a default, development friendly access log.

This is an implementation of a subset of the Apache mod_log_config syntax, a lot is still missing. The support can grow over time, but this is already better than the current solution that offer no customization at all. See http://httpd.apache.org/docs/2.0/mod/mod_log_config.html for reference.

Internally, it brings the following changes:
* logMiddleware is replaced by accessLogApacheMiddleware and accessLogJsonMiddleware
* tests have been added for accessLogApacheMiddleware and accessLogJsonMiddleware
* timerMiddleware now populates `request.Env["START_TIME"]`
* tests have been added to timerMiddleware
* recorderMiddleware now records `request.Env["BYTES_WRITTEN"]`
* tests have been added to recorderMiddleware
* gzipMiddleware has been moved down the middleware stack in order to get the correct `BYTES_WRITTEN` when the payload is compressed.