Change junit.framework.Assert and junit.framework.TestCase references to org.junit.Assert.

Fixes #106.
Use assertThat instead of assertTrue/False/instanceof and some other assertion changes
Go one step further with Matchers from the hamcrest-library
Review: change .size() usages to hasSize(), remove is() wrappers
Merge pull request #114 from TWiStErRob/assert

Use checkstyle to forbid JUnit 3 and some other internal packages.