Support for boolean HTML-attributes

[th0r]

Current v-attr implementation doesn't support boolean attributes (e.g. disabled).
Here is the common use-case:

<button type="submit" v-attr="disabled: !submittionAllowed">

It doesn't work because v-attr uses this.el.setAttribute method. So, when submittionAllowed equals to true, then submit button will look like this:

<button type="submit" disabled="false">

...which is treated by browser as disabled button.
My proposal is to remove current attribute if it's value is falsy (except zero).

[yyx990803]

Hi Yuriy, great suggestion. If you have time, could you please make sure tests are updated and passed according to the contribution guide? Thanks.

[coveralls]

Coverage remained the same when pulling 7bb8ef1 on th0r:boolean-attrs-support into c71c6a4 on yyx990803:master.