auto timestamp migration class names if auto_timestamp_class is set in configuration

This is implementation of the issue #306. I'm missing this feature too, it is even a blocker for me.

I've created a new configuration option named "auto_timestamp_class". When this is set to "yes", "true" or 1, the class name will be suffixed by the timestamp, which avoids conflict in names. If the configuration value is missing or "no", "false" or "0" it works as before (thus it is backward compatible)

I've chosen postfix instead of prefix because PHP class names must start with a letter or underscore.

I've also included a small fix to upper case the first letter of the migration name automatically, i.e. it is now equivalent
phinx create Abc
and
phinx create abc

I've tested the code, but I didn't run unit tests.

I think the `date()` call should be restricted to `Util` class for DRY reasons. The best thing to do would be to move `$className` generation into `Util` as well. See also #355.