First pass on supporting Apache log format

First, compat 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.

Then, the goal is to reused the well known Apache log formatting syntax to
define Go-Json-Rest access log.

The user can define his own record format, or pick a predefined one.

This is an implementation of a subset of the Apache mod_log_config syntax,
a lot if 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.
Implement %b content-length

The recorder middleware is in charge of collection that number.
The order of the middleware is rearranged in order to "record" after
gzipping.
Remove %b from the TODO
update the %b documentation
Add test for the timer and recorder middlewares.
Make sure the recorder records the bytes written with WriteJson.
Test the request start time
Test Gzip and Recorder at the same time ...

... and prove that '{"Id":"123"}' gzipped takes more space than non gzipped.
Typo
stub test file for the log middleware
Add test for the Apache log common format
Split the logMiddleware in two middlewares.

accessLogApacheMiddleware and accessLogJsonMiddleware
docstrings
Add tests for accessLogJsonMiddleware
Be a bit more defensive with the Env interface.

Not sure how this middleware will be used in the future.
Better check that the Env variable are populated.
Rename the log format contants
Fix %b and support %b (apache log format)
Merge pull request #98 from ant0ine/apacheLogging

Apache logging