have startDate/endDate for fairSale, no duration!

[martinkrung]

Start of a sale can't be in the future right now, becaus as soon as tx is done, sale starts

Describe the solution you'd like

Have startDate/endDate like in fixedprice sale!

https://github.com/cryptonative-ch/mesa-smartcontracts/blob/d41eac1ec4a80f7decf24b5b88e3169de16210b1/contracts/sales/FairSale.sol#L133

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Possible Implementation

Look how its done in fixedprice sale

[denciu]

Resolved by #130