Added spec file for package building under Centos, RHEL

This is a spec file for building package under RedHat based distros. Tested on Centos 6.4.
To build package do following

    git clone --depth=1 https://github.com/yandex-load/yandex-tank ~/rpmbuild/SOURCES/yandex-tank-<version>
    tar -czf ~/rpmbuild/SOURCES/yandex-tank-<version>.tar.gz ~/rpmbuild/SOURCES/yandex-tank-<version>
    rpmbuild -ba ~/rpmbuild/SOURCES/yandex-tank-<version>/yandex-tank.spec