Pending tests

This adds pending tests

```R
# unit test syntax
test_that("my test", {
  pending("hasn't been implemented")
})

# bdd syntax
describe("feature", {
  it("can have not yet tested specs") # implicit call to pending()
  it("can have not yet tested specs with partial implementation", {
    # some code
    pending("not yet done")
  })
})
```
This also modifies the summary reporter to display pending tests. I haven't changed other reporters.
I don't know if that the is smartest way to implement this, but it's a start :) I basically added a pending state to the expectation struct. 

What do you think? Does that makes sense for the package?