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 :)
This intentionally follows the semantics of `Array.prototype.indexOf`.

It can thus only search for an element of the array. If you wanted to search for a sequence of elements (bytes), you can supply a Buffer as the needle.