Merge pull request #11 from keenlabs/2.0.0

KeenClient-Java 2.0.0
remove outdated maven note in README
Fix bug in Android JSON handler

The Writer was not being closed, resulting in empty HTTP requests.
Build and publish a raw JAR for Android

This is in addition to the AAR which was already being published
Remove Core from dependencies in POMs
Update README
Update version to 2.0.0 release
Remove old note from README
Remove dead links from README
Update README wording
Fix wording in README
Change gradle.properties
Add KeenJsonHandler#writeJsonWithoutEncryption/readJsonWithoutDecryption()
Make KeenClient pass errorCode to the callback
Add ERROR_CODE_INVALID_EVENT
Set sourceCompatibility to 1.6
Enhance the error handling of encryption/decryption