Use `Get` variable to allow POE Fee to be customizable

[shawntabrizi]

substrate-poe-tutorial/runtime/src/poe.rs

Line 10 in 4231f30

const POE_FEE: u32 = 1000;

[shawntabrizi]

Like this:
https://github.com/paritytech/substrate/blob/master/srml/balances/src/lib.rs#L211

Also remember to expose it in the API:
https://github.com/paritytech/substrate/blob/master/srml/balances/src/lib.rs#L405