Added returns(), alias to is()

Created an alias to is() called returns() which allows for more concise and readable tests in the case of testing the return value of a method.

Example:
assertThat(object.calculateValue(), returns(5))

instead of:
int actualValue = object.calculateValue()
assertThat(actualValue, is(5))

which also reads more as natural language than:
assertThat(object.calculateValue(), is(5))