Sylius search

Hello Guys,

This is my initial pull request for the Sylius search feature as outlined on https://github.com/Sylius/Sylius/issues/1583

Within SearchBundle there is a README file with a few instructions on how to use the bundle. This explains the notion of the finder and how it can be used optionally with facets for different situations such as a front-end category page, the main search results page or through the backend interface. 

At this time I have implemented support for small applications using ORM (mysql actually) and larger ones with ElasticSearch. It should be easy to add more engines in future.

There are a few TODO comments in the code mentioning our decisions and what can be changed in the future, but I'm hoping for some feedback and guidance first.

Videos created for demo purposes:
http://youtu.be/mACQeN1UNzQ
http://youtu.be/XBfxdrlQyzM

Regarding Behat tests there are 2 modes, one for mysql and one for elastic search. Keep that in mind when you want to run them.

![screenshot 2014-08-12 15 20 58](https://cloud.githubusercontent.com/assets/1845809/3892445/61286522-2232-11e4-81dd-c9d4560270ec.png)

![screenshot 2014-08-12 15 56 39](https://cloud.githubusercontent.com/assets/1845809/3892447/6421eb04-2232-11e4-8130-2b847fe6fecf.png)


@agounaris Is it possible to override these price values from main config.yml? It seems these values always shows up in the search page along with the values defined in applications config.yml.

Is it possible to define how multiple filter on same  property will be applied? When I selected multiple price range it returned zero result. What I am asking for is ability to use `should` instead of `must` in bool filter for price.