Added factorlog for better logging

This patch will replace the defalt `log` with `factorlog`[1] which:

* Make us stop using `if h.debug {}` all the time as you can set the desired severity
* Support for colors ;-) (we can remove that just by using plain formatter)
* Support for different formatters
* It will add the filename and line number which is **extra** usefull when debugging.

The output looks like this:

```
[2014-04-17 13:53:19] [DEBUG:docker.go:32] Using docker socket: unix:///var/run/docker.sock
[2014-04-17 13:53:19] [DEBUG:build.go:128] Determining whether image test-ruby-app is compatible with incremental build
[2014-04-17 13:53:20] [INFO:build.go:75] Existing image for tag test-ruby-app detected for incremental build
[2014-04-17 13:53:20] [DEBUG:build.go:140] Performing source build from git://github.com/pmorie/simple-ruby
[2014-04-17 13:53:20] [DEBUG:build.go:265] Saving build artifacts from image test-ruby-app to path /tmp/sti721042307/artifacts
[2014-04-17 13:53:21] [ERROR:build.go:325] Couldn't attach to container
[2014-04-17 13:53:21] [ERROR:build.go:334] Error while waiting for artifact save in incremental build (2)

```