Check compiled code against java6 api signature

Usage of the animal-sniffer-maven-plugin now avoids introducing code that
depends on a newly java7 (or later) api
Enforce dependencies do not require java 7+

This done with the maven enforcer plugin with the enforceBytecodeVersion
codehaus extra rules
Format to use spaces instead of tabs
Merge pull request #87 from redfish4ktc/ensure-working-java6-build

Ensure working java6 build