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.
The following example searches for...