add support for reading key from remote url

[lusis]

This adds support for reading encryption keys from a remote url (similar to my existing cookbook). Not sure how you wanted to handle error so I made something up as argument errors. Also didn't see any functional tests for the encryption stuff so I just ran the existing spec. Here's a sample session output:

[] me@~/development/chef-repo$ knife data bag show --secret-file http://localhost:5678/ephemerals/my_company/decrypt test_edb passwords
ERROR: ArgumentError: Remote key not available from 'http://localhost:5678/ephemerals/my_company/decrypt'
[] me@~/development/chef-repo$ knife data bag show --secret-file http://localhost:5678/ephemerals/my_company test_edb passwords
ERROR: ArgumentError: Remote key not found at 'http://localhost:5678/ephemerals/my_company'
[] me@~/development/chef-repo$ knife data bag show --secret-file http://localhost:5678/ephemerals/my_company/decrypt test_edb passwords
id:     passwords
mysql:  MySexyPassword
[] me@~/development/chef-repo$

[seth]

I merged this in 52ab7df which also includes a bit of refactoring, some tests, and a fix on top.

Note: s[-1] == "\n" is not what you think.