Make facet expressions API consistent with filter expressions.

There is now `FacetExpr`, analogous to `FilterExpr` which provides
a nice little DSL for creating facet expressions.

Examples:
`FacetExpr.stringAttribute("color").terms()`
`FacetExpr.price.rangesMultiSelect(r1, r2).selected(r2)`

Compared to current version:
`new FacetExpressions.Terms("color")`
`new FacetExpressions.Price.RangesMultiSelect(list(r1, r2), r2)`

The DSL is implemented as a thin layer on top of the expression
classes, so the examples shown above are simply syntax sugar
for the code shown below them.
Add documentation for facet expressions.
Add mising varargs.
Merge remote-tracking branch 'upstream/master'
Add documentation for filter expressions.
Merge branch 'master' of github.com:mkonicek/sphere-play-sdk into mkonicek-master
Update changelog for release.