Make dependency on Spring optional

[jan-zajic]

I wrote a json config parser to make a dependency on ignite-spring and even whole spring stack unnecessary. It's only required to configure ignite from xml config file, so it's better to make it optional. In vertx it's best practice to configure things via json, so i wrote parser that replace this xml file (see my update of documentation) and made it default. Users can still use xml file if add dependency to ignite-spring, also documented.

[agura]

@jan-zajic I think it makes sense but JSON configuration should be part of Apache Ignite project. Otherwise provided JSON parser will be outdated soon.