Support Multiple Datasets in a Single Report

For Issue #102 

The basic idea is to support multiple tables in a single report.  For example, a customer info table at the top and all the customer's orders below that.

```sql
-- Customer Info and Order History
-- VARIABLE: {name: "customer_id"}

-- @dataset true
-- @title Customer Info
SELECT * FROM customers WHERE customer_id = "{{customer_id}}";

-- @dataset true
-- @title Order History
SELECT * FROM orders WHERE customer_id = "{{customer_id}}";
```

This pull request implements this feature for MySQL, MongoDB, and PHP report types and maintains backwards compatibility.  

The OPTIONS header was a mix of report-level options and table formatting options.  I kept the report-level options as part of the OPTIONS header and split the other ones into a FORMATTING header.  Specifying everything in the OPTIONS header will continue to work, but is deprecated now.

The pull request also updates the CHART, FILTER, and ROLLUP headers to enable you to specify which dataset the header applies to.  

Finally, the pull request adds multi-dataset support to all the report formats (except CSV).  For example, XLS reports will have multiple sheets, one for each dataset.