Move build_from_file to a factory class

This `build_from_file` method is constructing a new `Contract` and is not
related with the other methods in the module `Contracts`.
The idea is to separate what is construction of a `Contract` from what is
using the contract.
The way the gem will be used has to be changed from: `Contracts.build_from_file(path, host)` to: `Contracts::ContractFactory.build_from_file(path, host)`