put eventsource in its own package

Currently, because it is declared in `package http`, when I use this package I import it as

```go
import (
    eventsource "github.com/antage/eventsource"
)
```

Is there a reason not to name the package `eventsource`? I could change my import statements to

```go
import (
   "github.com/antage/eventsource"
)
```

Not a big deal, but it is more expected.