[WIP] ORM Style Interface

[shawndeprey]

This PR is intended to add an ORM style interface for all Recurly models. Additionally, to support the ORM style lookups, auto-configuration based on environment variables has been added for ease of use.

Example:

# Singular Lookup
Recurly::Account.find_by(id: 'wr9sz399pmt6')

# Plural Lookup
Recurly::Account.all.each # ...
Recurly::Account.where("limit = ? and order = ? and sort = ? and begin_time = ? and end_time = ?", 25, 'desc', 'updated_at', 2.days.ago, Time.current).each # ...

# Not filters
Recurly::Account.where("order = ? and sort = ? and state != ?", 'desc', 'updated_at', 'inactive').each # ...
Recurly::Coupon.where("order = ? and sort = ? and coupon_type != ? and state != ?", 'desc', 'updated_at', 'bulk', 'expired').each # ...