Include more information in request string representation

## The Problem

Typhoeus does not override the default implementation for `to_s` on the `Typhoeus::Request` object.

## The Motivation

The default string presentation of `Typhoeus::Request` objects is generally unhelpful:

```ruby
request = Typhoeus::Request.new('http://localhost')
request.to_s
=> "#<Typhoeus::Request:0x007fdb91c58a18>"
```

One may want to log the request, but the only way to do so presently is to log each "piece" of the request individually. A contrived example:

```ruby
Logger.info("Request: url=#{request.url} options=#{request.options}")
```

It would be pretty cool if one could get helpful information by logging the request itself:

```ruby
Logger.info("Request: #{request}")
```

1. Simpler logging.
2. Possibly more robust (assuming a good string representation)

## The Solution

I didn't feel qualified to make decisions about how to best represent a `Typhoeus::Request` as a string, so I went with the simplest solution of delegating the call to Object#inspect.

Let me know what you think! I'm happy to adjust the implementation, rebase, etc. as needed.