Added in base line exception classes.
Added machine level error handling for detecting cURL errors and reporting on them correctly.
Added server level error handling.
Added proper error handling at the library level. This basically identifies when the polymorphic item fetching methods come up empty and throws a 404 resource not found exception.

I also fixed a bug where doing single item fetch by its key would never work correctly.
Updated README to reflect the added error handling.
Deprecated the getSectionByKey method from Library.php and added in a polymorphic getSection method, which makes it possible to get a section by its key or by its title.