Add assertThat() short-cut methods on Truth

Add `assertThat()` short-cut methods on `Truth`, and implement a test to ensure that no `TestVerb.test()` methods are added without a companion `Truth.assertThat()` method accompanying it.

Note: Internally, the Google core libraries team engaged in discussions on our internal java mailing lists, and it was determined that many people found `ASSERT.that()` disruptive and shouty, and since `assert()` is a non-starter (java keyword), a less disruptive option of `assert_()` was chosen.  However, this resulted in some code that looked like this:

```java
assertTrue("We're all about junit", someCondition);
assert_().that(fooCollection).has().allOf("a", "b", "c");
assertEquals("a", someString);
```

This was seen as disruptive to the reader.  This commit provides a Hamcrest/FEST-like alias for assertions against the test verb (that is, short-circuiting failure strategy).  It results in code looking like this:

```java
assertTrue("We're all about junit", someCondition);
assertThat(fooCollection).has().allOf("a", "b", "c");
assertEquals("a", someString);
```

This style introduces a conflict with statically importing hamcrest or FEST assertions, but `Truth.assert_().that()` will remain available (as will `TruthJUnit.assume().that()`).  Users can set project policies around what style they prefer as appropriate for their organization or context.
Because this code is then less deviating form TestVerb.  It was pulled from there, and I wanted to reduce the diff.  But it's not that big a deal, and I can inline.