AWS Credentials support, StaticCredentials Provider (#17)

* added credentials API, disabled FFI backend for now

* Added CredentialsTest

* Added support for specifying CMAKE_BUILD_TYPE on the command line

* Added support for AwsCredentials and StaticCredentialsProvider

src/AWS/CRT/Internal/FFI.php

@@ -1,51 +0,0 @@
-<?php
-
-namespace AWS\CRT\Internal;
-
-use \Exception;
-use \RuntimeException;
-
-/**
- * @internal
- * Forwards calls on to libaws-crt-ffi via FFI
- */
-final class FFI {
-    private static $ffi = null;
-    private static $refcount = 0;
-
-    function __construct() {
-        if (is_null(self::$ffi)) {
-            try {
-                self::$ffi = \FFI::cdef(file_get_contents(__DIR__ . "/../../../api.h"), __DIR__ . "/../../../libaws-crt-ffi.so");
-                self::init();
-            } catch (Exception $e) {
-                throw new RuntimeException('Exception while initializing CRT via FFI', 0, $e);
-            }
-        }
-        ++self::$refcount;
-    }
-
-    function __destruct() {
-        if (--self::$refcount == 0) {
-            self::clean_up();
-            self::$ffi = null;
-        }
-    }
-
-    /**
-     * Forwards any call made on this object to the FFI function of the
-     * same name with the supplied arguments. Argument type hinting and checking
-     * occurs at the CRT wrapper.
-     */
-    function __call(string $name, $args) {
-        return call_user_func_array(array(self::$ffi, $name), $args);
-    }
-
-    private static function init() {
-        return self::$ffi->aws_crt_init();
-    }
-
-    private static function clean_up() {
-        return self::$ffi->aws_crt_clean_up();
-    }
-}