Add library version and os info to user agent String.

To help debug from the server side,  library version and device/os
info are added to user agent. The user agent is used for all the
http requests issued by HttpRequests.

The basic user agent is defined in properties file as:
user.agent=CloudantSync/@version@, where "@version@" replaced by
current build version in build.

Informaton about os.name, os.arch and os.version are always included in
user agent string no matter running on Android or not

String: "Android" + Android codename + Android API level are
included only when running on Android
Merge pull request #4 from cloudant/24238-user-agent

Add library version and os info to user agent String