Create multi-module project

[lhazlewood]

JJWT 0.10.0 will support different JSON and Base64 implementations and allow users to plug-in their own implementations where necessary.

As a result, all 3rd-party libraries should be moved to individual jjwt modules so users can depend on only what they need and not pull in more classes or dependencies than are required.

This issue is to track the work to:

• Change the existing pom.xml to reflect a jjwt-root pom artifact
• split existing code into a number of modules:
  • jjwt-api: contains classes and interfaces users will add as a compile-time dependency to their project
  • jjwt-impl: contains classes and interfaces users will add as a runtime dependency in their project. This artifact will contain private implementation-only classes and interfaces that users should never depend on in a project and where semver is not guaranteed and can change at any time.
  • jjwt-jackson: contains Jackson implementation for JSON (de)serialization. Only needed as a dependency if you use Jackson in your project
  • jjwt-orgjson: contains org.json implementations for JSON (de)serialization. Only needed as a dependency if you want to use org.json in your project. Android users will want this since it is natively supported since Android version 1 and does not require any additional 3rd party dependencies.

[lhazlewood]

Completed via #351 - merged to master, will be released for 0.10.0