Add SafeToken level

[AshiqAmien]

The aim of this level is to teach the user how to decompile EVM bytecode, as well as call functions without an ABI. The instance contract uses the ABI of untampered source code, while the backdoored source code is deployed. The steps to complete the level is as follows:

• Decompile the contract to find the backdoor function, and notice that it's linked to the ERC20 transfer function.
• Activate the backdoor by calling the function with the function signature, along with the encoded secret string as a parameter.
• Call the now-backdoored transfer function to drain the owner's tokens.

I'd appreciate any feedback - I'm open to any suggestions for changes and improvements to the level.

[xaler5]

Rebased in here
#547