def withdraw(coin, params = {})
      raise ConfigurationError.new('No coin type specified') unless coin
      raise ConfigurationError.new('Invalid coin type specified') unless Coin.valid?(coin)
      request(:post, "/#{coin}_withdrawal", params)
    end
def withdraw(coin, params = {})
      raise ConfigurationError.new('No coin type specified') unless coin
      raise ConfigurationError.new('Invalid coin type specified') unless Coin.valid?(coin)
      request(:post, "/#{coin}_withdrawal", params)
    end
Withdrawal of the specified coin type.

 coin    - The coin type
 amount  - The amount to withdraw.
 address - The coin type's address we will send the amount to.