`SafeERC20` doesn't make sense

[ItsNickBarry]

What does "safe" mean? Any of the following:

• revert if an ERC20 function returns false but does not itself revert
• use increaseAllowance and decreaseAllowance instead of approve
• workaround nonstandard alternatives to increaseAllowance and decreaseAllowance
• workaround tokens that don't even conform to ERC20 and don't return a bool at all

#239 and #245 were an attempt to make the library consistent with the first point above, because it's the only point that is truly the responsibility of the contract and makes sense within the context of a single transaction. But consistency comes at the cost of not pandering to nonstandard tokens.