New coverage check implementation. 

New report APIs to check coverage, support for coverage checks in Ant,
rework Maven check goal implementation based on new APIs.
Do not overload setters as Maven configuration will not properly work
with this.
Improve documentation on how to specify coverage ratios.
Update examples to new check options.
Fix config syntax in example.
Merge pull request #106 from jacoco/issue-106

Coverage check redesign incl. Ant support.