Force older versions of cURL to verify server SSL certificate, update readme, add module version to the User-Agent header

This pull request does the following:

## 1. Update readme

It adds a note about the dependencies and their installation to the README. It also adds a note about the default socket transport and the lack of SSL certificate validation and encourages people to use libcurl transport.

## 2. Force older versions of cURL to verify server SSL certificate

In versions of cURL prior to 7.10, `CURLOPT_SSL_VERIFYPEER` option was disabled by default. This meant that by default, curl didn't validate the certificate authenticity against a local CA bundle.

This pull request explicitly sets this option to `1` to force SSL certificate validation with older versions of cURL and mitigate possible MITM attacks.

For explicitness and consistency sake, this branch also sets `CURLOPT_SSL_VERIFYHOST` option to `2` which is a default value.

References:

* http://curl.haxx.se/libcurl/c/curl_easy_setopt.html#CURLOPTSSLVERIFYPEER
* http://curl.haxx.se/libcurl/c/curl_easy_setopt.html#CURLOPTSSLVERIFYHOST

## 3. Append module version to the User-Agent header