Add RpmHelper with tests

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Add get_requires to SpecFile

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Add install_build_dependencies() method

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Check build dependencies when using rpmbuild (Fixes #23)

Signed-off-by: Tomas Hozza <thozza@redhat.com>