Rename 'environment' config option & add singletons setup

[nventuro]

This PR ended up being sort of involved :/ It does a number of things:

• it renames the environment configuration option to contractAbstraction, since e.g. it may make sense to use truffle contracts in a non-truffle environment, unlike the old naming suggested
• it upgrades to a newer version of contract-loader which is used by the singletons module, allowing for customization of the default sender and gas
• it provides configuration options for the default sender and gas of contracts returned by singletons

[frangio]

I really like the new subsections in Configuration.

[frangio]

LGTM!