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.

The android version (if running on android) or desktop os info
are also appended to the user agent string.