Adding .rubocop.yml with some minor customizations

Excludes files that already have some cleanup to do, or aren't actually maintained by us.

Also customizes the style guide to allow some things that are good (hash rockets, compact class/module nesting), okay in some circumstances (nested conditionals), or just don't matter one way or the other (`raise` vs. `fail`).
Note that this is about this: http://www.refactoring.com/catalog/replaceNestedConditionalWithGuardClauses.html

I think "prefer guard clauses" is good, but in simple case, the following seems fine to me:

```ruby
def my_method(some_bool)
  if some_bool
    do_thing
    do_second_thing
  end
end
```

This seems as good or better than the other options:

```ruby
def my_method(some_bool)
  return unless some_bool
  do_thing
  do_second_thing
end
```

```ruby
def my_method(some_bool)
  do_thing if some_bool
  do_second_thing if some_bool
end
```