Added @ConditionalOnPropertiesPresent condition

Added @ConditionalOnPropertiesPresent that enables conditional bean creation when all required properties are defined.

Inspired by example shown during "Multi Environment Spring Applications" talk at SpringOne2GX 2013 by @joshlong (http://www.youtube.com/watch?v=3l5awQGo8XA&t=56m39s). I find this @Conditional very useful and I believe it's in general useful enough to become a part of Spring Boot distribution.