Build: shared libraries and versioning

These changes cause shared libraries to be built by default. In
particular, lib/libopenvwitch.so, lib/libsflow.so,
ofproto/libofproto.so, and ovsdb/libovsdb.so will be built. Original
behavior of building static objects excusively may be accomplished by
providing the --disable-shared argument to configure.

Additionally, versioning is introduced to each of the libraries objects
paving the way for APIs to be built around them. A detailed comment
outlining the rules for changing a version number is provided in
configure.ac. Note that at this time, the version number is set to
1.0.0, no API is specified yet, and there are no requirements to maintain
any sort of compatibility in any of the libraries.

Signed-off-by: Scott Mann <smann@noironetworks.com>