Improvement changes

Changes.

* Rename ``getHeaders()`` to ``getDefaultHeaders()``
* Throw an exception if missing data provider when generating the excel.
* Implement ``toFile()`` method for save the report on server.
* Set report name correctly on ``toBrowser()`` method.
   Now when ``toBrowser($reportName)`` is called the report name is generated by taking the report name and the file extension from methods.

Issues #2 and #3 done.