Update CHANGES.md for 1.8.0

Update README.md and CHANGES.md for 1.7.1

Update README.md for lz4-pure-java

Update CHANGES.md for 1.6.0

Update README.md and CHANGES.md for 1.5.1

Update README.md and CHANGES.md for 1.5.0

Fix a link to CHANGES.md

Fix the link to Maven Central

Issue #53 -Fixed calling flush on closed LZ4BlockOutputStream

Update README documentation versions.