add support for Alchemy TextExtractDates endpoint

[davismat]

The is an endpoint for relative date extraction:

/text/TextExtractDates

params include, for example:

BasicNameValuePair outputModeParam = new BasicNameValuePair("outputMode", "json");
BasicNameValuePair textParam = new BasicNameValuePair("text", input);
BasicNameValuePair anchorDateParam = new BasicNameValuePair("anchorDate", anchorDateStr + " 12");