Add example for optimistic concurrency support

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+This example shows how to use optimistic concurrency in ODB.
+
+The example consists of the following files:
+
+person.hxx
+  Header file defining the 'person' persistent class. Besides the standard
+  persistent class pragmas, this definition also uses the 'optimistic'
+  pragma to indicate to the ODB compiler that the class must support
+  optimistic concurrency. It also uses the 'version' pragma to specify
+  which data member will contain the object version.
+
+person-odb.hxx
+person-odb.ixx
+person-odb.cxx
+person.sql
+  The first three files contain the database support code and the last file
+  contains the database schema for the person.hxx header.
+
+  These files are generated by the ODB compiler from person.hxx using the
+  following command line:
+
+  odb -d <database> --generate-schema person.hxx
+
+  Where <database> stands for the database system we are using, for example,
+  'mysql'.
+
+database.hxx
+  Contains the create_database() function which instantiates the concrete
+  database class corresponding to the database system we are using.
+
+driver.cxx
+  Driver for the example. It includes the person.hxx and person-odb.hxx
+  headers to gain access to the persistent classes and their database support
+  code. It also includes database.hxx for the create_database() function
+  declaration.
+
+  In main() the driver first calls create_database() to obtain the database
+  instance and persists a sample 'person' object. It then emulates the
+  parallel execution of two processes that try to concurrently update or
+  delete this object. For each step the driver prints the versions of the
+  object as seen by each process.
+
+To run the example we may first need to create the database schema (for some
+database systems, such as SQLite, the schema is embedded into the generated
+code which makes this step unnecessary). Using MySQL as an example, this
+can be achieved with the following command:
+
+mysql --user=odb_test --database=odb_test < person.sql
+
+Here we use 'odb_test' as the database login and also 'odb_test' as the
+database name.
+
+Once the database schema is ready, we can run the example (using MySQL as
+the database):
+
+./driver --user odb_test --database odb_test