Add @ConditionalOnJava.

Added a new @ConditionalOnJava annotation that allows to conditionally
enable configuration based on the Java version that is running. 

The annotation currently supports two modes of restricting Java versions:
the default mode checks for a Java version equal or better than the
requested one. Beyond that it can be configured to only match if Java
version is older than the configured one.
Auto configuration for Jackson's JodaTime and JSR-310 modules.

We now register the Jackson JodaTime module with Jackson ObjectMappers
if it is on the classpath. We also register the JSR-310 module if it's
on the classpath and the application is running Java 8 or better.

Extracted the Jackson specific configuration previously residing in
HttpMessageConvertersAutoConfiguration into a JacksonAutoConfiguration
class.

Added the Jackson JSR-310 module as a managed Boot dependency.