[migrations] add parameters to skip excluded or removed table from migration
[migrations] add optional comment for migration
Merge pull request #582 from jaugustin/migrations-exclude-tables

Migrations : skip excluded or removed tables
Configuration system refactor - step 1

Configuration system refactor, according to issue 285:
- Add support for various configuration formats
- Create a more rational configuration hierarchy
- Validate the configuration values

Step 1: add new classes and relative tests
Configuration system refactor - step 2

Implement Propel\Common\Config\PropelConfiguration class to validate configuration
and load defaults, via Symfony\Component\Config\Definition\Builder\TreeBuilder class.
Remove all not more used options and define a better property structure.
Add the possibility to define some properties at runtime (i.e. passing some values
from command line)
Configuration System Refactor - Step 3

Refactor all classes, tests and fixtures which use configurations.
Add support for environment variables.