log 関数の挙動

It's niceeeeeeeeee gem!!

```ruby
require 'dydx'
include Dydx

cos(x)
# => #<Dydx::Algebra::Set::Cos:0xba6f99b4 @x=:x>
cos(x).to_x
# => "cos( x )"

log(x)
# => ArgumentError: wrong number of arguments (1 for 0)
log = Dydx::Algebra::Set::Log.new(x)
# => #<Dydx::Algebra::Set::Log:0xba992c34 @f=:x>
log.to_s
# => "log( x )"
```

現状， log 関数はこのようにしか扱えないようですが，これは仕様なのでしょうか？