multiple changes working towards issue #9

* using eclipse-aether
* using JSR-330
* upgrading dependencies
* requiring maven 3.1.0 or greater
* adding new RepositoryDownloader for downloading from a maven repo
jcabi-maven-slf4j no longer needed, fixes #21
adding tests for ArtifactBuilder
adding tests for RepositoryDownloader
plugin and dependency updates
changing the group id
renaming MAVEN_CENTRAL to REPOSITORY
Merge pull request #7 from klieber/issue/9_maven_central

Use maven central instead of downloading binaries from googlecode. Closes #9