Add Documentations.

Hello. I add more documents (yard) throughout the project.

@davy Could you please take a look that will these changes break anything you're using? Thanks.

**Code changes:**

I found unused helpers module (Removed in bc44856) and one unused method: `as_action?` (Removed in 57c6c61). Also I update `STDOUT` to `$stdout` (921da62) to make it more consistent.

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I have found two incorrect documentations. But could not solve it.

`Benchmark::IPS::Report` and `Benchmark::IPS::Job` has incorrect documentations in instance variable (`attr_reader`):

**`Benchmark::IPS::Job` incorrect instance variable document:**

![screenshot 2014-06-20 12 10 25](https://cloud.githubusercontent.com/assets/1000669/3336469/d6e6c6b8-f830-11e3-8485-e550ee6b3ff4.png)

Relevant changes: https://github.com/JuanitoFatas/benchmark-ips/blob/8c0c32148f4c95bd0e787bd4e0cfcc325ebe61f3/lib/benchmark/ips/job.rb#L4-L15

The descriptions is wrong: `class Benchmark::IPS::Job.`. 

**`Benchmark::IPS::Job` incorrect instance variable document:**

![screenshot 2014-06-20 12 11 00](https://cloud.githubusercontent.com/assets/1000669/3336474/e87907d8-f830-11e3-9bf5-44132af0fd21.png)

Relevant changes: https://github.com/JuanitoFatas/benchmark-ips/blob/6df96fedbef199d16f15e2a1329d620ed2a9ab5a/lib/benchmark/ips/report.rb#L8-L9

The descriptions is wrong: `class Benchmark::IPS::Report.`.