Enable Dynamic Smith Config

[DylanGriffith]

It would be useful to have a way of starting smith without needing a .smithrc. This would mean that require 'smith' would not raise if it couldn't find a '.smithrc', but perhaps could still try. A client may do something like:

require 'smith'
Smith.configure(my_smith_options)

[filterfish]

Smith now has a sensible default config and the rest can be set using environment variables. The only config that needs to be set are:

• agency.acl_directories
• agency.agent_directories

which can also be set using the environment variables:

• SMITH_ACL_DIRECTORIES
• SMITH_AGENT_DIRECTORIES

respectively.