"conflict" does not work for non-trivial constraints

This file (made up to illustrate the problem)
```
{
	"require": {
		"symfony/symfony": "*"
	},

	"conflict": {
		"symfony/symfony": "< 2.2.0, >= 2.3.0-dev"
	}
}
```
... should install the best 2.2 Symfony release. At least @igorw briefly confirmed on IRC that it should behave as one would expect.

The problem is that the version parser will generate a MultiConstraint for the "conflict" entry. In turn, the ```RuleSetGenerator``` will [find all packages](https://github.com/composer/composer/blob/master/src/Composer/DependencyResolver/RuleSetGenerator.php#L167) that match this constraint and add those as conflicting targets. 

Obviously, no package will match < 2.2 and >=2.3 and so no conflict rule will be created.

Here's a quick proof of concept patch that shows that turning around the MultiConstraint into a kind of disjunctive constraint solves the issue, i. e. (currently) installs Symfony 2.2.3: https://gist.github.com/mpdude/5825777.

The problem is that this change would prevent you from having e.g. ```>= 1.0.0-dev, < 1.2.0-dev" to say your package is incompatible with the 1.0 and 1.1 series of something. (A single clause like "< 1.2.0-dev" is not an issue.)

Another possible solution that would probably require some (major?) schema/parser extension would be 
```
   "conflict": { 
       "symfony/symfony": [ "< 2.2.0", ">= 2.3.0-dev" ]
   }
````
... only for the "conflict" key? Does not sound good to me either.