Introduce top-level Gradle build

 - Convert ethereumj-studio build from Maven to Gradle as well
 - Extract common build elements to top-level build.gradle
 - Update top-level README accordingly
Move Gradle wrapper infrastructure to top level
Support running ethereumj-studio as executable jar

See updated ethereumj-studio/README.md for details. Note that this
commit removes the `build-post-package.xml` ant file (for now) to
simplify the Gradle arrangement. This functionality can be re-introduced
later and more simply using Gradle's native DSL. In the meantime, both
the -core and -studio projects can be run using `gradle run` or run as
executable jars by running `gradle shadowJar` followed by the usual
`java -jar` execution of the resulting `-all` jar file found in each
project's respective `build/libs` directory.
Update .gitignore settings for Eclipse output dirs
Remove unused imports