Factorize unittest code

Add DatabaseTest and ReadOnlyDatabaseTest abstract classes
create \Payutc\Log
Update Paybox to use \Payutc\Log
Update MADMIN to use \Payutc\Log
Remove class/Log.class.php
Fix bugs in Log class

Remove require config.inc.php
Fix second parameters of array_key_exists in __callstatic
Add unittests for logger
Use SlimExtras LogWriter

The default LogWriter of Slim is too basic. This LogWriter allow
you to specify a file and prefix the log message with the level
and the date. This can be configured of course.
I also put a default configuration in config.inc.dist.php
Merge branch 'master' into unittests-user
Add unittests for User and Blocked

Add some interfaces to construct test cases (DatabaseTest and
ReadOnlyDatabaseTest).
Update travis.yml
Correct indentation
Merge pull request #123 from trecouvr/logger

Logger
Merge pull request #127 from trecouvr/unittests-user

Add some unittests for User and Blocked