request object impl. for show/list funcs evsukov89/couchbase-lite-ios#1

few fields were not implemented:
* cookie - need to check how well NSHTTPCookieStorage plays with
  NSURLRequest's URL
* form - don't have time to make form parsing atm
* info – certain fields were set to 0 because they require quite
  expensive db request but doesn't bring much of a value

this will be eventually addressed