Import X-Ray Agent Plugin project

[willarmiros]

Description of changes:

• Added X-Ray Agent project in plugin form
• Migrated all build logic to Gradle, verified artifacts are created correctly and tests pass with gradle clean build
• Prepared the packages to be published to maven central, and verified the publishing will work by staging a release
• Removed the sample app subproject, which will be replaced with a more proper sample app in the future

This leaves the repo in an awkward, duplicated state, and I plan on removing all the legacy Maven subprojects in the near future. I'll also update travis later, the Gradle project isn't very testable by CI until Disco is released to maven central. Just didn't want to make this PR more bloated than it already is.

Might not be in the incredibly near future, but I plan on adding license, checkstyle, null checker, etc to keep parity with the SDK standards.

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[willarmiros]

Travis is failing because it's still testing the old, maven-based project. When Disco and 2.7.0 of the SDK are in maven central, I'll switch travis over to the new project.