Using Base64 instead of DatatypeConverter

[azagniotov]

This PR addresses issues #317 & #297 and #143 (points out a significant difference between the DatatypeConverter & Base64)

Replacing usages of javax/xml/bind/DatatypeConverter with java.util.Base64

Please note

This PR removes support for building the library against Java 7 as it leverages java.util.Base64 which was added in Java 8. I do not know what the maintainers of jjwt have in mind in terms of sunsetting support for Java 7, so I am not sure if my PR is the right direction. If support for for Java 7 will be dropped, then the issue #308 will be addressed too

As an alternative, the following PR #283 tries to solve this issue by adding a dependency on jaxb-api, which brings in DatatypeConverter without dropping the Java 7 build support

[coveralls]

Coverage remained the same at 100.0% when pulling 801ff37 on azagniotov:issue_317_replace_datatype_converter into 44faaca on jwtk:master.

[marcospereira]

Add oraclejdk10 and oraclejdk11 here would be good. We can allow failures for oraclejdk11 by adding the following configuration:

matrix:
  allow_failures:
    - jdk: oraclejdk11

[azagniotov]

I tried adding oraclejdk10 and oraclejdk11, but build failed on Travis when running tests against these JDKs with NPE (https://travis-ci.org/jwtk/jjwt/builds/398495426).

For now, I removed the above changes. If/when the current PR gets merged, we can issue a separate PR to add the oraclejdk10 and oraclejdk11 to Travis config

[lhazlewood]

JDK 7 may be dropped in the 1.0 final release, but until then, I think we need to continue support for JDK 7.

Based on this, are you willing to re-issue the PR and pull in the jaxb lib as a dependency for JDK 7 environments?

[azagniotov]

@lhazlewood I can do that. Thanks

[lhazlewood]

@azagniotov please disregard my previous comment - check out #333 for the latest on how we're trying to solve this.

[lhazlewood]

Closing in favor of #341. Thanks for your continued help and review on this @azagniotov !