Add Entry class to represent entries in the .netrc.

This makes it possible to read login and password like this:

```ruby
  netrc = Netrc.read
  entry = netrc['machine']
  login = entry.login
  pass  = entry.password
```

Through the use of a lightly-tweaked Struct, the Entry object behaves
almost identically to the old arrays, including including implicit
splatting when it is on the right side of an assignment:

```ruby
  netrc = Netrc.read
  entry = netrc['machine']
  login, pass = entry
```