Add initWithCoder and refactor common code into initWebView

In newer versions of Xcode, if the "Instantiation: Prefer coder" check box is ticked for a .nib or .xib files, custom view instances get initialized with initWithCoder: instead of initWithFrame:, which in the case of ACEView causes silent failure, as the web view is not initialized.

This pull request adds an initWithCoder: method and factors out the common webView initialization code into a separate method.