Fixes and updates.

Added:
- Instructions of extension usage.
- Compatibility function.

Edited:
- Description as a part of instructions.
- Small fixes of requirements and names.
- Language fixes.
- Timer fix (as it starts from 60 automatically, the first update should be to 59).
- Keywords fix.

As for mobile view I have two screenshots to compare.
![cron_status_responsive_compare](https://cloud.githubusercontent.com/assets/7805635/3787519/d48045a0-1a1f-11e4-98ef-149984133e6f.png)
The left screenshot is the **current responsive version**.

Advantages:
- The standard look (same as default phpBB ACP tables).
- Can be shown on really small devices even when the name of Cron task is quite long.

Disadvantages:
- It is not a table look as we can see when using PC.

On the right there is a screenshot of the **non-responsive version**.

Advantages:
- Similar look for PC and mobile devices.

Disadvantages:
- Can't be shown on really small phones.
- Long names of Cron tasks will break the design.
- It is a non-standard look.

So **the conclusion**:
we can transform our Cron Status page to look the same on PC and any mobile devices, but it isn't a good idea for some reasons.
I think that the current look of Cron Status page is good enough for all devices.