Add slither-prop tool: Automatic generation of unit-tests and Echidna properties

[montyly]

Features:

- Generate properties based on given scenarios. This PR includes ERC20 properties, for the  following scenarios
   - Transferable - Test the correct tokens transfer
   - Pausable - Test the pausable functionality
   - NotMintable - Test that no one can mint tokens
   - NotMintableNotBurnable - Test that no one can mint or burn tokens
   - NotBurnable - Test that no one can burn tokens
   - Burnable - Test the burn of tokens. Require the "burn(address) returns()" function
- Truffle support:
   - Generation of the migration file
   - Generation of unit tests files. For each scenario, one file test the correct initialization, and the second one contains the actual tests
- Echidna support

[montyly]

What is missing to merge this PR:

• Handling of throw/revert in unit tests
• Tests and user documentation. The documentation might be directly written for building-secure-contracts

On the long run, we will add

• Other ERCs,
• New scenarios,
• Support for other platform (embark/ etherlime, ...)