Better WP HTTP API Use

* Set `sslverify` to `false`, to be sure cURL works in any case.
* Check if request was successful and ther run image import.
* It's better to do json_decode before the loop.
* Ref: #1.
Improve Settings Pages

* options.php handles "Settings Saved" message by default.
* Use `screen_icon()` to display icons.
* Ref: #3
Refactor render_msg

* Fix PHP notices.
* Make message texts I18n ready.
* `msg_text` propery was used only inside this functon, replaced
with local variable.
* Ref: #3
Merge pull request #2 from Mamaduka/master

Improved WP HTTP API Usage