Allow Jexl expressions on CrawlDatum's fixed attributes

CrawlDatum allows Jexl expressions on its metadata fields nicely, but it lacks the opportunity to select on attributes like fetchTime and modifiedTime.

This includes a rudimentary date parser only supporting the yyyy-MM-dd'T'HH:mm:ss'Z' format:

Dump everything with a modifiedTime higher than 2016-03-20T00:00:00Z

bin/nutch readdb crawl/crawldb/ -dump out -format csv -expr "(modifiedTime > 2016-03-20T00:00:00Z)"

Dump everything that is an HTML file

bin/nutch readdb crawl/crawldb/ -dump out -format csv -expr "(Content_Type == 'text/html' || Content_Type == 'application/xhtml+xml')"

Keep in mind:

• Jexl doesn't allow a hyphen/minus in field identifier, they are transformed to underscores
• string literals must be in quotes, only surrounding qoute needs to be escaped by backslash