Add function to get earliest date for each source

[ahalterman]

Each source goes back a different length of time. We need to know what date coverage starts on.

<?xml version="1.0" encoding="utf-8"?>
<soap:Body xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <GetSourceDetails xmlns="http://getsourcedetails.source.services.v1.wsapi.lexisnexis.com">
    <binarySecurityToken>*****</binarySecurityToken>
    <sourceId>408508</sourceId>
    <includeSourceElement>true</includeSourceElement>
  </GetSourceDetails>
</soap:Body>

Then we need to get the date from the "sourceguide":

...
FILE-NAME: DSTONL
COVERAGE: From April 30, 2013 through current
COVERAGE-TYPE: Full-text
FREQUENCY: Daily; Monday - Sunday
UPDATE-SCHEDULE: Same day
...