refactor(API): Introduce FxSelector and FxSelectorContext.
fix(API): Add license headers.
Merge pull request #165 from hastebrot/refactor-fxselector

refactor(API): Introduce FxSelector and FxSelectorContext.
Merge branch 'master' into feat-typesafe-matchers
refactor(API): Retrieve NodeFinder in FxAssert from FxSelectorContext.