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

Added InvalidAlgorithmError class, and reorganised to follow psr-4 layout for autoloading as composer package.

For example...
```
"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/"
    }
  }
```