Trusted proxies can be read from config file

[jtuttle]

Create a new class Conjur::ConjurConfig using the Anyway Config gem and configure it to:

• read from /etc/conjur/config/conjur.yml
• read environment variables prefixed with CONJUR_
• fall back to TRUSTED_PROXIES for backwards compatibility
• default to empty string if no values are configured

Update Conjur::TrustedProxyFilter to read the trusted proxies value from the ConjurConfig object.

Logging

• INFO to Rails log when config object created "Loaded config from "
• WARNING to Rails log when no config file present "No config file found at "

Unit Tests

• update the Conjur::TrustedProxyFilter unit tests to use the ConjurConfig object
• create new unit tests for Conjur::ConjurConfig to test the precedence behavior