Add url_fopen option checking to requirements

[dmgawel]

The plugin uses stream_wrapper_register with STREAM_IS_URL flag, thus requiring allow_url_fopen to be enabled. It is a common security practice among hosting providers to disable this option as a precautionary measure. In such a setup the user only sees following error:

Unable to create directory. Is its parent directory writable by the server?

This PR adds a check during plugin init and provides appropriate error message when allow_url_fopen is disabled.

Related issue: #325

[joehoyle]

Great, thanks @dmgawel