Add support to Jar tasks on Gradle Build

[jeandersonbc]

The current Gradle support covers all original compilation tasks and test execution.
Another important task that must be covered, and it is between compile and test is the build task.
The build task is responsible for generating some important jars.

[jeandersonbc]

According to my python script that compares the Ant and Gradle execution, I'm missing two jars:

$ python3 main.py
Running command "ant clean build"
Finished in 18.53 secs
Running command "./gradlew clean buildJars --info"
Finished in 9.52 secs
Total files: 1839
Common files: 1836
Missing files: 3
 - /Users/jeandersonbc/projects/jpf/jpf-core/build/RunJPF.jar
 - /Users/jeandersonbc/projects/jpf/jpf-core/build/RunTest.jar
 - /Users/jeandersonbc/projects/jpf/jpf-core/build/classes/java/nio/package-info.class

I'm working on those two missing jars.
I don't know why the gradle build is not copying the package-info.class file into the build directory, but I believe this is not critical right now.

[gayanW]

package-info.class seems to go in jpf-classes.jar

[cyrille-artho]

Looks like it is done, but it's good to have a test that depends on build.properties being in the right place. You can make a new issue for that, in which case I can close this one.

[jeandersonbc]

Sure! I'm opening a new issue for that. Since changes are merged, I think it's ok to close this one now.