Don't crash if a custom/non-standard HTTP status code is encountered

When using POSTMan with an API that uses custom (non-standard) HTTP status code (which is completely legal according to [RFC2616](http://tools.ietf.org/html/rfc2616#section-6.1.1)) the extension currently tries to access an undefined field when creating the `responseCode`:

    'detail':httpStatusCodes[response.status]['detail']

Since `httpStatusCodes` only contains standard codes, the request for `detail` causes an error (can't access a property on `undefined`). This PR tries to fix this.

I also added `*.iml` to `.gitignore`, since this is the other format used by IDEA for project files which is already featured in the file with `.idea`.