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.
Yes, you are absolutely right, that some stuff is missing here. The complete product search implementation will be done in multiple iterations. So one of the next steps is to create meta model classes so that developers can explore through classes and methods for which you can search and analyze for. So they won't need to consult the HTTP API documentation and write strings.