Moved default location constraint logic from config loader to endpoint

This will allow `Jekyll::S3::Uploader` to be run programmatically while providing a sane default that you'd get from a `yml` config file.

I cannot get the full test suite to run properly, but running the affected specs, tests pass:

```sh
adelevie@Alans-MacBook-Air:jekyll-s3 → master rspec spec/lib/endpoint_spec.rb 
.

Finished in 0.00105 seconds
1 example, 0 failures
adelevie@Alans-MacBook-Air:jekyll-s3 → master rspec spec/lib/config_loader_spec.rb 
..

Finished in 0.00567 seconds
2 examples, 0 failures
```

Notice that I completely removed a test from the config loader spec and moved its equivalent to a new file, `endpoint_spec.rb`.