Added JUnit Tests for CardDAO. Minor modifications to CardDAo based on
the results of JUnit Tests.

To run tests, please make sure the following jars included in the
classpath:

${SDK_ROOT}/lib/testing/appengine-testing.jar
${SDK_ROOT}/lib/impl/appengine-api.jar
${SDK_ROOT}/lib/impl/appengine-api-labs.jar
${SDK_ROOT}/lib/impl/appengine-api-stubs.jar