Add basePath config parameter

[ppalaga]

Adds basePath config parameter defined as follows:

    /**
     * The base path under which the repository will be served.
     * <p>
     * By default, {@code org.acme:my-artifact:pom:1.2.3} will be served under
     * {@code http://localhost:<port>/org/acme/my-artifact/1.2.3/my-artifact-1.2.3.pom}.
     * <p>
     * If {@code basePath} is set to e.g. {@code foo/bar} then {@code org.acme:my-artifact:pom:1.2.3} will be served
     * under {@code http://localhost:<port>/foo/bar/org/acme/my-artifact/1.2.3/my-artifact-1.2.3.pom}.
     */
    @Parameter( property = "mrm.basePath", defaultValue = "/" )
    private String basePath;

[ppalaga]

Fixed via #94