Implemented async callbacks to PHP regardless of thread model, Sigv4 signing (#24)

* Updated to get latest aws-crt-ffi

* Added implementation for signing_result

* Added main thread queue and callback queueing to signing process

* Updated to get support for date on signing_config_aws

* added date support to SigningConfigAWS, fixed up reference issues on http message signing

* Broke C code into separate files, moved common stuff to php_aws_crt.h

* Use a unity build to make PHP do the right thing with our lib

* Updated aws-crt-ffi to v0.1.3 to get sigv4 changes

* Added CRT::isAvailable for safe runtime detection of CRT

* Fixed up test resource safety, added tests to prove it

* Added CI jobs for every version of PHP on linux (#26)

composer.json

@@ -15,9 +15,10 @@
     },
     "minimum-stability": "alpha",
     "require": {
+        "php": ">=5.5"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5"
+        "phpunit/phpunit":"^4.8.35|^5.4.3"
     },
     "conflict": {
         "phpunit/php-timer": ">=2"