add namespace macros

Hope you don't mind this change. This is purely for selfish reasons. By using these macros and the pre-processor I can fool my editor that there isn't any extra scope -> No extra level of indentation for the code within the namespace block!

I also think it's better with self documenting code
```c++
END_NAMESPACE(leveldown)
```
instead of commenting the code like this:
```c++
} // namespace leveldown
```