Delegate all Makefile targets to ant

Embed and use ant-contrib so that it's possible to build on systems
without ant-contrib on the default classpath.
ant: Use ant-contrib jar directly
Rephrasing CONTRIBUTING re: branch for PRs
Making API doc links point at release version
Merge pull request #762 from rleigh-dundee/ant-makewrap-dev50

Delegate all Makefile targets to ant
ant: Default all properties and handle empty vars in conf.py
ant: Correct property defaults
Merge pull request #767 from rleigh-dundee/ant-propertydefaults-dev50

ant: Default all properties and handle empty vars in conf.py
Merge pull request #764 from hflynn/CONTRIB-update

Rephrasing CONTRIBUTING re: branch for PRs
Merge pull request #765 from hflynn/release-api-links

Making API doc links point at release version