Code cleanup and some minor functionality

* Split helper functions into helper.go
* Added Probability of having desired income function
* Added some comments
Deleted GPL from source
Further cleanup and fixed starting incomes
Fixed some bugs:

Line 21 simulation.go - (1-eff_tax) not eff_tax
Lines 73-80 path.go - Fixed Factors() to not return first ret year
Lines 66-68 retcalc.go - removed printing from RunIncomes()
taking a week off sitting at home