Batch array: assert elements are objects

[vweevers]

Closes #138.

Breaking change, major.

[juliangruber]

For reference, pre-allocating the array is the fastest method for big and small arrays

[ralphtheninja]

I just noticed that we're calling back on the same tick on multiple places. Creating new issue for this.

[vweevers]

@ralphtheninja also, my first instinct was to throw (because it's a programmer's error), instead of calling back with an error. But then I thought it should be the same as the lines below:

abstract-leveldown/abstract-leveldown.js

Lines 173 to 174 in a5069de

	if (err = this._checkKey(e.type, 'type'))
	return callback(err)

What's our policy on that?

[vweevers]

Never mind, I realized we do that in order to wrap the error in levelup.