NUTCH-1867 CrawlDbReader: use setFloat to pass min score

git-svn-id: https://svn.apache.org/repos/asf/nutch/trunk@1629539 13f79535-47bb-0310-9956-ffa450edef68
Joint commit for NUTCH-1868 Document and improve CLI for FileDumper tool and NUTCH-1869 Add a flag to -mimeType fiag to FileDumper

git-svn-id: https://svn.apache.org/repos/asf/nutch/trunk@1629941 13f79535-47bb-0310-9956-ffa450edef68