Make structure of classes compatible with PSR-4

[andergmartins]

For our libraries we use the pattern PSR-4.
In short, instead of the internal classes be located at ./includes and each file having the "Embed" prefix, we should create a "embedpress" folder inside the library dir, and rename the files:

includes/EmbedPress.php --> library/embedpress/Core.php
includes/EmbedPressLoader.php --> library/embedpress/Loader.php,

etc.

Another change will be to use PHP namespaces for that classes:

Alledia\EmbedPress\Loader for example, instead of EmbedPressLoader

That allow us to use the composer autoloader to load the classes without the need to include each one.