DoubleClickValidator refactoring

- Refactor DoubleClickValidator to not depend on OpenRTB;
  move it from doubleclick-openrtb to doubleclick-core
- Also, provide DoubleClickMetadata.URLConnectionTransport
DoubleClickValidator reviews
OpenRtbMapper, build reviews

- Added missing two methods in the mapper interface
- No need anymore to checkout the openrtb project for building
- Depend on openrtb 0.6.4-SNAPSHOT
Merge pull request #8 from opinali/master

Validator and mapper reviews