Adding fields which are listed as optional in the spec to support har file read from tools which are outputting those fields.

I should have had this in the last commit, but hadn't fully looked at the spec, just noted fields which were being output by the Chrome Developer Tools, and subsequently causing the import to fail.

The one item I am not certain about is the connection field in HarEntry. The spec is somewhat ambiguous to me. It seems like it could be a int, but it also states it can be any unique connection id, hence the decision to make it a string (thinking someone could utilize a UUID or similar).