Forbid to write to http upstream anything but string

Http proxy assumes that an application responses with headers in first chunk and packaged strings in rest of chunks. So application must not send to upstream anything but string.
'get_service_async' is asynchronous now.

Now get_service_async returns future<Service> that will be ready when the connection to service is established.
storage.write no longer copies a value to write

Method 'write' of storage stub no longer copies a value to write. Also it's possible to write cocaine::io::literal, not only strings.
Update version in *.spec file
Remove unused code and fix building with c++ 4.7.

'method_handler_traits' isn't used in code and doesn't compile with g++4.7, so i have been removed it. Also copy/move constructors/assignment operators have been added to http_headers_t and http_request_t, because g++ 4.7 doesn't implicitly generate assignment operator for http_headers_t.
Release 0.10.6.2.
Merge branch 'master' into v0.10