Fix /reports date filter: make 'All Time' filter work #91

Basically this just allows empty values for the from and to fields.
Allow date filters with only 'from' or 'to' value, not both

* Allow just 1 of from or to filters, don't require both.
* Check filter params are not empty, not just if they're set
  Avoids defaulting to filter by 1970-01-01
Merge pull request #1020 from rjmackay/date-filter-fixes

Date filter fixes from uchaguzi