Fix QuickBuilder classTargets initialization

Defining the classTargets as null (build all) or as an empty array (do
not build any classes) isn't the same thing.
Merge pull request #616 from K-Phoen/fix-quickbuilder-targets

Fix QuickBuilder classTargets initialization
allow custom collection in ObjectFormatter
Throw an exception when no connection has been defined for a database (fixes #620)
Merge pull request #622 from K-Phoen/issue-620

Throw an exception when no connection has been defined for a database (fixes #620)
[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)
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