Allow configs with indifferent access

[pilaf]

Currently FmRest's configs require symbol keys. Since these configs are often loaded from YAML files though it would be good if we could support indifferent access (e.g. strings OR symbols for keys).

E.g., currently this fails:

# fmrest_config.yml
host: foo.com
database: Database

FmRest.config = YAML.load_file("fmrest_config.yml")

When trying to connect with the above example the gem will complain that it can't find :host in the config hash, etc.

Note that YAML does support symbol keys (just prefix : to each key), but that's kinda ugly and not great to require it.

BTW, we don't want to use Rails' HashWithIndifferentAccess.

[pilaf]

Addressed with inclusion of FmRest::ConnectionSettings.