Remove expand! in favour of EmbiggenedURIs

[mudge]

GitHub: https://github.com/altmetric/embiggen/issues/1

As discussed in #1, rather than having two expansion methods (one gracefully handling errors but not communicating any information about them and one eagerly raising exceptions), consistently return a sum type from expand that both encapsulates exceptions and communicates success:

uri = Embiggen::URI('http://examp.le/badlink').expand
#=> #<Embiggen::EmbiggenedURI http://examp.le/badlink>
uri.success?
#=> true or false
uri.error
#=> #<Embiggen::BadShortenedURI ...>
uri.to_s
#=> 'http://examp.le/badlink'

Embiggen::EmbiggenedURI delegates to an internal Addresssable::URI but does so explicitly through Forwardable rather than SimpleDelegator: some methods are delegated (e.g. host, inferred_port, request_uri) but only for convenience and the true way to get a URI instance is to use Embiggen::EmbiggenedURI#uri.

[mudge]

Some questions for reviewers:

• EmbiggenedURI is a bit of an unwieldy name: perhaps we should go for something simpler like Embiggen::Result?
• Refactoring after removing expand! led me to extract an EmptyLocation Null Object but it currently just sits in lib/embiggen/uri.rb; is it worth extracting into its own file?

[mudge]

Discussed in a group:

• It's unnecessary to swallow all errors raised during expansion, we could make handling too many redirects, etc. a client responsibility (as they truly are an exception, as opposed to expanding an unshortened URL which is just part of the contract);
• We should return types that we directly control or are part of the standard library so return a URI instead of an Addressable::URI if possible;
• There was agreement that seeing all the hops during expansion would be useful but perhaps via a different method, e.g. trace (then expand could be implemented in terms of trace.last).