Don't reject full document in case of invalid metadata
From original PR #17 from @fcamblor

If you try to index a document with an invalid metadata, the full document is rejected.

For example:

```html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html lang="fr">
<head>
<title>Hello</title>
<meta name="date" content="">
<meta name="Author" content="kimchy">
<meta name="Keywords" content="elasticsearch,cool,bonsai">
</head>
<body>World</body>
</html>
```

has a non parseable date.

This fix add a new option that ignore parsing errors `"index.mapping.attachment.ignore_errors":true` (default to `true`).

Closes #17, #38.