User-Agent header improvements

1. Currently the default User-Agent header is of the form `createsend-ruby-{version}`. Modify this to include Ruby version and platform information: `createsend-ruby-{version}-{RUBY_VERSION}-p{RUBY_PATCHLEVEL}-{RUBY_PLATFORM}`.
    So the default user agent would look more like:
    ```ruby
    => "createsend-ruby-3.2.0-1.9.3-p392-x86_64-darwin12.3.0"
    ```

2. Allow the default User-Agent header to be overridden by doing this:

    ```ruby
    CreateSend::CreateSend.user_agent "my custom user agent"
    ```