Install headers and change protoc_lib to just protoc

To use protobuf for further compilation of different projects, we usually need files in standard locations - .h files in /usr/local/include etc. 
The proposed patch adds copying .h files to install directory.

It also changes the name of protoc_lib library to simple protoc (in distributions you usually have libprotoc.a or protoc.lib, this library is found by cmake, for example).

Currently after using this great cmake-based building toolset user usually need to copy headers and rename library manually (see for example
https://gist.github.com/DennisOSRM/23c17c4cb0d890f00cb7#file-build_other-bat
from OSRM
https://github.com/Project-OSRM/osrm-backend/wiki/Windows-Compilation )

Be sure to check this under Linux too :)