Add client_lib_id into user-agent of all http requests

[dimitrystd]

We get more and more traffic for FileAPI v2 and looking for an option how to monitor traffic from connectors for quick reaction to errors. The simplest and convenient for all solution to use "User-Agent" header field. The current sdk already has client_lib_id (introduce in #19) but it was added for another reason (indicate link between particular uploaded file and client lib version).

DoD:

• SDK adds client_lib_id and version as "User-Agent" for all http requests (doesn't mater GET\POST\DELETE)
• If you want to keep the original "User-Agent" too, then add information in the begging of "User-Agent" . An example:
  • The original user-agent: Guzzle/5.3.0 curl/7.35.0 PHP/5.5.9-1ubuntu4.14
  • The new user-agent: smartling-api-sdk-php/2.0.2 Guzzle/5.3.0 curl/7.35.0 PHP/5.5.9-1ubuntu4.14
• User-agent string has format {client_lib_id}/{version}
• Changes are merged to 2.0.0-g5 and 2.0.0 branches

[dimitrystd]

Reopened. Such implementation will work only for file upload. All other commands with send smartling-api-sdk-php instead actual client (drupal; WP). Just make 2 properties in BaseApiAbstract. They will be used for all requests and one more time for Upload. We used this approach in java and .net sdks