buffer: add indexOf method

Adds a `.indexOf` method to `Buffer`, which borrows semantics from
both `Array.prototype.indexOf` and `String.prototype.indexOf`.

`Buffer.prototype.indexOf` can be invoked with a Buffer, a string
or a number as the needle.  If the needle a Buffer or string, it will
find the first occurrence of this sequence of bytes.  If the needle is
a number, it will find the first occurrence of this byte.

Fixes #95.

If you're unsure about or not happy with the semantics, let's discuss :)
If we go with a hybrid interface, I also think different specialised functions would be a cleaner approach.