Minor gradle fixes.

* Changed Statsd to StatsD to conform with naming standard of project.
 * Added findbugs and jacoco plugins.
 * Made the jacoco plugin run on build.
 * Made the javadoc task run on build.
 * Explicitly specified the tool version for checkstyle and used an up-to-date checkstyle file.
 * Added default task.
 * Updated version to 2.4.0-SNAPSHOT in light of recent changes.
 * Split the slf4j dependency so that under compile it only depends on slf4j-api.
 * Test still depends on slf4j-jdk for simplicity.
 * Changed to junit:junit:4.11 from junit:junit-dep:4.11, since with 4.11
junit-dep has been deprecated.
 * Added the findbugs annotations and JSR305 annotations.
 * Enabled Xlint:all on compile.
 * Made the wrapper version explicit.