Added option to access non-localized fields with locale
Added option to access non-localized fields with locale

This commit adds a work-around to access fields that are non-localized
while using a non-default locale. In specific, this commit adds two new
options:

- For fields accessed via Entry::all(), a new third parameter can be
  used to avoid the InvalidArgumentException. This might be the more
  useful behaviour, but could would break existing code and it therefore
  defaults to the old behaviour.
- For fields accessed using get, the new helper method
  Entry::isFieldLocalized may be used to quickly verify whether the
  locale should be set for this call.

This implements a non-breaking work-around for #270

contentful/contentful.php,contentful/contentful.php,contentful/contentful.php