optionally return results as literals

[elrayle]

Fixes #16 Optionally have ldpath return RDF::Literals instead of Strings

DEFAULT remains returning results as string or the specified data type

Also...
* remove pin of bundler
* add example results to README

[jrochkind]

Should there be docs somewhere documenting the new param and what it does? Either method-level comment docs on the methods that take the new arg, or README, or something?

I had trouble figuring out what it did at first looking at the code -- is it the difference between returning a String (?) and an RDF::Literal object? Should we think about if there's a clearer name for that then maintain_literals, since once it's released it'll be hard to change? (There may not be a clearer name).

[elrayle]

Added the expected result should be for documentation purposes.

[jrochkind]

That documents the normal case, but the thing being added in this PR with maintain_literals is still (I think?) entirely undocumented. As the simplest possible thing, since ldpath doesn't seem super documented in general anyway but that's not a problem for this PR, what about just adding antoher examples to the README with maintain_literals: true, and what that would return?

Also, just confirm you think maintain_literals is the right name of this arg? Preferable to, oh, use_literals or return_literals or return_rdf_literals or rdf_literals: true or something like that? I think it's up to you, I just think it's good to at least publicly consider if it's the best one we can think of before locking it into a release from which it can't be changed without backwards incompat.

I'm not totally locked into any of these ideas, just trying my best to give a responsible non-rubberstamping review on code I'm not actually at all familiar with. Disagreement welcome you can just tell me "nah, I know this code better and it's right how it is", and I'll just approve! Thanks!

[jcoyne]

Should this be same_type? Should it be a private method?