allow custom collection in ObjectFormatter
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)
Allow the same behavior to be registered multiple times
Only allow the id parameter for behaviors which accept it
Configuration System Refactor - Step 3

Refactor all classes, tests and fixtures which use configurations.
Add support for environment variables.
update schema XSD definition to allow for specifying a column's singular form
Use Column->getSingularName() where applicable
Introduce Column->getPhpSingularName() and use it where appropriate. The
phpSingularName property of Column is initialised using the new
'phpSingularName' attribute for columns in the schema, or (if not specified in
schema) generated based on the column's phpName.

Minor update of comments to remove grocer's apostrophe (possessive form is
"its" rather than "it's")
don't tell lies about the origin of the auto-generated value
Expand ColumnTest to include testcases for new singular form phpName feature
Merge pull request #527 from cristianoc72/config-refactor

Added new configuration system
Merge pull request #617 from havvg/master

allow custom collection in ObjectFormatter
Merge pull request #638 from sjvs/features/override-column-singular-form

Introduce attribute to override singular form of a column's phpName
Merge pull request #635 from K-Phoen/behavior

Allow behaviors to be applied more than once
Fixed #633 - Added `hasConnectionManager` to serviceContainer.
Fix issue #669
Merge pull request #670 from cristianoc72/issue-669

Fix issue #669
toArray with includedForeignObjects should not prefix the arrays of foreignObjects
Fixed yaml test