Added InvalidAlgorithmError class, and reorganised to follow psr-4 layout

[58bits]

Added InvalidAlgorithmError class, and reorganised to follow psr-4 layout for autoloading as composer package.

For example in composer.json, you can add the following to autoload the class.

"repositories": {
    "http-signer": {
      "type": "package",
      "package": {
        "name": "gwynne/http-signer",
        "version": "1.0.0",
        "source": {
          "url": "https://github.com/dgwynne/php-http-signature",
          "type": "git",
          "reference": "7abfde5fc7ad29b62446fb59c967458f48227220"
        }
      }
    }
  },

"autoload": {
    "psr-4": {
      "Gwynne\\": "vendor/gwynne/http-signer/src/"
    }
  }

composer install would then automatically clone the repository into the following vendor directory...

── composer.json
├── composer.lock
└── vendor
    ├── autoload.php
    ├── composer
    └── gwynne
        └── http-signer
            ├── LICENSE
            ├── README.md
            └── src
                └── HTTPSignature.php

Using the signature class then becomes...

require 'vendor/autoload.php'; // Loads all vendor related 'requires'.

$date = gmdate(DATE_RFC1123);
$headers = array('date' => $date);

Gwynne\HTTPSignature::sign($headers, array(
    'key' => '46653e78ce9df4f2d9ae93gft5f5c281',
    'keyId' => '18KF2FGK6807ZQA9FGT4',
    'algorithm' => 'hmac-sha1'
));