Adding support for attribute values with pixels

[atiqsayyed]

fixed #167

[hrj]

Oh, this became too generic a fix. I misunderstood your question yesterday; sorry.

You need to make it a bit more specific.

1. The change should be applied only to those attributes that specify a length.
2. Also the value of the attribute itself shouldn't change; only the use-site should be changed. That is, where the attribute gets interpreted as a length.

Ensuring 2 will automatically ensure 1.

And finally, all of this needs cross-checking with the specs and / or browser.

[atiqsayyed]

@hrj not sure why the build is failing

[hrj]

There's some problem with the communication protocol. I restarted the build manually and it passed!

Can you rebase these changes onto master? Ping me on gitter if you need help with that.