JDK9 PNG Writer Backport

This is a backport of the standard ImageIO PNG writer available in Java 9, which includes a very important configurability and performance enhancement. This library can be used on both Java 7 and Java 8.

Prior to Java 9, the ZLIB deflater used internally by PNGImageWriter always used the BEST_COMPRESSION compression level, which tries to achieve optimal file sizes regardless of the performance penalty. This behavior was not configurable.

In Java 9, the ZLIB deflater used by PNGImageWriter uses a more sane compression level default (4), and allows the compression level to be customized with the standard ImageWriteParam compression attributes.

This library contains and automatically registers this improved PNG writer with the ImageIO system, directing the ImageIO service registry to prioritize it over the built-in standard PNG writer.

Many thanks to Laurent Bourgès for implementing this Java 9 enhancement to begin with.

###Building

gradlew check: Compiles and runs all quality checks, including the unit tests.
gradlew jar: Builds the JAR file.
gradlew uploadArchives: Deploys to Maven Central (requires a modified gradle.properties file).