lib: Add new header <openvswitch/version.h> to versioning info

Exposes the package version as string and the library versioning
as numeric macro.

OVS_LIB_VERSION may be used to check for a particular version of
the interface.

OVS_LIB_REVISION and OVS_LIB_AGE exist to map a set of installed
headers to a particular shared library instance.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>