Parser is bad Alternative

`Alternative` is expected to be a monoid.

But for `Parser` we have:

``` haskell
import Prelude
import Data.Attoparsec.Text
import Control.Applicative

λ> parseOnly (fail "oops") "" == parseOnly (fail "oops" <|> empty) ""
False

λ> parseOnly (fail "oops") ""
Left "Failed reading: oops"
λ> parseOnly (fail "oops" <|> empty) ""
Left "Failed reading: empty"
```

Implications:
1. Adding `empty` erases useful error message.
2. User can't substitute `p1 <|> p2 <|> p3` with `asum [p1, p2, p3]` (`asum` always appends `empty` at the end).