Remove dependencies jackson-databind, jacoson-annotations

OpenRtbJsonFactory used Jackson's ObjectMapper only to enforce an
optimization (JsnFactory associated to an ObjectMapper seems to
perform better caching of reflection). Drop this; caller can
configure the OpenRtbJsonFactory with an ObjectMapper-bound
JsonFactory if they want. Also, more standard practice with Jackson.
Update Jackson to 2.4.1.1
Make getJsonFactory() public, create it on demand
Merge pull request #1 from opinali/master

Dependency trimmings and updates