Add VersionManager abstractization (breaking change)

[decebals]

See #154 for motivation.

[coveralls]

Coverage increased (+0.07%) to 54.794% when pulling e3ff846 on version_manager into 49bd1f5 on master.

[decebals]

This PR comes with a VersionManager interface:

/**
 * Manager responsible for versions of plugins.
 *
 * @author Decebal Suiu
 */
public interface VersionManager {

    /**
     * Check if a {@code constraint} and a {@code version} match.
     *
     * @param constraint
     * @param version
     * @return
     */
    boolean satisfies(String constraint, String version);

}

The default implementation is DefaultVersionManager and it uses SemVer. You are not forced to use SemVer in your projects. If you want something custom, then override createVersionManager method from AbstractPluginManager and supply your implementation.

Note This PR comes with a breaking change.

[janhoy]

Probably a good change. Did a quick look only.

[coveralls]

Coverage increased (+0.07%) to 54.794% when pulling 80cf2b3 on version_manager into 49bd1f5 on master.