fix(client): Allow leading/trailing whitespace on email addresses

@zaach - mind reviewing?

This is comprised of 2 commits, the first fixes the leading/trailing space on email problem, the second standardizes the parameter name for an element to `el` instead of a mix of `el`, `which` and `selector`.

fixes #519 
Not sure if we care anymore, but in a previous commit (https://github.com/mozilla/fxa-content-server/commit/2b5bf630553b413fe0c0f70a849dc2f24097053e), we did a custom `trim()` method because of "IE 8 doesn't support `String.prototype.trim`".

Spoiler alert:
```js
// IE 8 doesn't support String.prototype.trim
function trim(str) {
  return str && str.replace(/^\s+|\s+$/g, '');
}
```

Elsewhere, [/app/scripts/lib/fxa-client.js](https://github.com/mozilla/fxa-content-server/blob/cdc16f433157b37fc254e7dc6d33b5f4f6b43cf4/app/scripts/lib/fxa-client.js), we seemingly use jQuery's `$.trim()`:
```js
function trim(str) {
  return $.trim(str);
}
```