Add possibility of using as decorator

[podusowski]

trafficstars

What do these changes do?

You can use timeout as function decorator.

Are there changes in behavior for the user?

Users get additional API

Related issue number

Fixes https://github.com/aio-libs/async-timeout/issues/141

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes
• [x] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov[bot]]

Codecov Report

Merging #142 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #142   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          105       110    +5     
  Branches        15        15           
=========================================
+ Hits           105       110    +5     

Impacted Files	Coverage Δ
async_timeout/__init__.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update fd3c1eb...1dfe0d5. Read the comment docs.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 3 committers have signed the CLA.

:white_check_mark: asvetlov
:x: Piotr Podusowski
:x: podusowski

---

Piotr Podusowski seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}