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.
Any reason to add this instead of just using the constant?