Added new module equity-core for defining common utilities to all child
projects. Products of this package should have no other dependencies on any
io.vos.equit packages. Created module with mvn command:

$ mvn archetype:generate \
      -DgroupId=io.vos.equity \
      -DartifactId=equity-core \
      -DarchetypeArtifactId=maven-archetype-quickstart
Adds global dependency in the parent POM on guava.
Implements the Specification pattern as interface Specification, abstract base
class BaseSpecification and concrete implementations AndSpecification,
OrSpecification, NotSpecification, TrueSpecification, and FalseSpecification

see http://martinfowler.com/apsupp/spec.pdf for more info
Adds -Xlint:all flag to maven-compiler-plugin arguments.