Ignore exception if its ancestor is an ignored class.

When I add an ignored class, my expectation is that all subclasses
will be equally ignored. This makes it easy, for example, to ignore
all 4xx code HTTP response exceptions by specifying one generic
superclass.
Merge pull request #118 from ecin/ignore-ancestor-chain

Ignore exception if its ancestor is an ignored class.
Add note about how to handle custom errors in Sinatra

The custom error handlers halt execution, so the error will not reach
Bugsnag.
Fix reference in readme
Merge pull request #122 from RKushnir/patch-1

Fix reference in readme
Merge pull request #121 from kytrinyx/document-sinatra-errors

Add note about how to handle custom errors in Sinatra
Update HC
Accept both String and Class instances as an ignored class.

If we accept actual Classes, developers can avoid typo-related
bugs if they so desire, e.g.:

```ruby
configuration.ignore_classes << "NotFoundEror"
 # => didn't catch a typo, sadface

configuration.ignore_classes << NotFoundEror
 # => Ruby will complain it can't find a constant
```