Add set_calc_mode() to set calcMode and calcId

This commit introduces the new method set_calc_mode() to allow user to
set calculation mode and calcId explicitly. It enables user to work
around the save file propmt which was reported as issue #100. It also
removes the 'fullCalcOnLoad' attribute and set the 'calcOnSave'
attribute to 0 by default.
Merge pull request #104 from schnell18/master

Add set_calc_mode() to set calcMode and calcId.
Merge branch 'master' of github.com:jmcnamara/excel-writer-xlsx