New build system based on Gradle

[admin]

Until now, this project used Apache Maven to build the code (thanks to the XWiki project for suggesting that at that time). However, that build required to manually install JCLF before executing Maven (due to it not being available from Maven Central), and I received user feedback about the current build processes across the Carte ecosystem (EchoSVG and Carte face similar issues) being unacceptable.

Gradle lags behind Maven in a few aspects (like parallel testing), but overall is more powerful and flexible than Maven, and much less restrictive about artifacts that do not come from Maven Central. In consequence, I decided to migrate the build systems of the io.sf.carte projects to Gradle. Both CSS4J and EchoSVG have been migrated already, and the top-level Carte is in the works.

The release procedure is much simpler now, as the Gradle build takes care about things like copying the jar files to a jar directory, or even converting to CRLF the line endings of the top-level text files, which previously had to be done separately. Deployment to the bare-bones Maven repository that this project uses was also made trivial by Gradle (that's another separate script that was removed).

This project's README.md at Github has been updated to reflect that, as well as the usage guide.

I hope that this improves the user experience a bit.