support for configurable User-Agent http header

Instead of the "Restlet/<version>" as value for User-Agent http header, the jdbc drivers uses as default "<title>/<version>". title and version are loaded from MANIFEST.MF which is populated by pom.xml's <name> and <version> tag.
Optionally a JDBC property "userAgent" can used. If so, the resulting User-Agent string is "<jdbcprop> via <title>/>version>", e.g. "Grails Driver 2.0.0-M02 via Neo4j JDBC Driver/2.0.3-SNAPSHOT".
Merge pull request #11 from sarmbruster/useragent

support for configurable User-Agent http header