Require PHP version 5.3.3 or newer in composer.json file

Composer allows the minimum PHP version to be defined for a project. Since the README file indicates that PHP version 5.3.3 or newer is required, this should be declared in the composer.json file.
Update dependencies via Composer

Since the composer.json file has been modified, a composer update command should be run. This updates the composer.lock file, which locks the project to specific versions of dependencies.
Update README to describe installation of Guzzle via Composer

Guzzle is defined as a dependency in the composer.json file, and as such it should be installed via Composer.
Fix list item numbers
Fix markdown syntax
Merge pull request #4 from cloudant/php-composer

Utilize Composer Features