Product search

This is a draft and we want to add a meta model in the future, so that you can easily create sort, facet and filter expressions for product projections without the need to concatenate strings.

A start for a documentation can be found here:

http://sphereio.github.io/sphere-jvm-sdk/javadoc/product-search/io/sphere/sdk/meta/ProductSearchDocumentation.html

@lauraluiz please review. If you are done, ask @gogregor to also review.
Shouldn't there be more on filters and facets? At least the information from the HTTP API doc (http://dev.sphere.io/http-api-projects-products-search.html#search-filters, http://dev.sphere.io/http-api-projects-products-search.html#search-facets). Laura's presentation includes internal mongodb and elasticsearch details which should not really matter here. It should only be about API functionality and performance.