Reword failure message for #delegate_method

Currently it says something like this:

    Expected Person#name to delegate to Person#company as #company_name

However, this isn't in line with proper terminology. It doesn't make
sense to say that one method delegates to another; you delegate a method
to another object. So now it says:

    Expected Person to delegate #name to #company object as #company_name

Additionally, we now clarify that #company (in this example) -- or
rather the object returned by #company -- is the delegate object, and if
this object never receives any method calls we say so. Instead of this:

    Calls on Person#company: ...

we say:

    Method calls sent to Person#company: ...

Furthermore, the description (which is shown as the tests are run) was
not correct when `delegate_method` was used against a class rather than
an instance. As an example, the following code:

    it { expect(Person).to delegate_method(:name).to(:company) }

would print out as

    should delegate #name to #company object

and now it prints out as

    should delegate .name to .company object