Add permit feature to token contract

[varasev]

This PR adds permit feature to the token contract.

[varasev]

Travis shows some issue with ethUtil package. I will take a look.

[varasev]

@k1rill-fedoseev Could you please take a look at one of the unit tests related to DSR. Travis caught the following mismatch:

657 passing (28m)
  1 failing
  1) Contract: ForeignBridge_ERC20_to_Native
       chai token
         Zero DSR
           should allow to executeSignatures after pay interest:
      AssertionError: expected '7400000000000000004' to be less than '7400000000000000000'
      + expected - actual
      -7400000000000000004
      +7400000000000000000
      
      at Context.it (test/erc_to_native/foreign_bridge.test.js:2207:66)
      at process._tickCallback (internal/process/next_tick.js:68:7)