Upgraded Guzzle from 3.4.* to 5.0.*

This is a major update. Changelog:
- Created Erpk\Harvester\Client\Request class. It is wrapper for new
Guzzle request classes, however it is backward compatible with old
Guzzle 3.
- Sessions are now saved to 2 files *.sess and *.cookies.
- Introduced new shortcut methods in Request class API:
disableCookies(), markXHR(), setRelativeReferer(), allowRedirects()
- Renamed Guzzle\Http\Exception\ClientErrorResponseException to
GuzzleHttp\Exception\ClientException
- Class holding request query doesn't have fluent API now. All
references have been fixed.
- Fixed rare bug with blank page while logging in
- Fixed rare bug in country module
- Removed NetworkInterfaceProxy
- ...and some more minor bugfixes and improvements