Docs: improvements of IO's document and website config

[alexandru]

Changes:

• in README moved the badges on a newline otherwise it leads to weird effects and it's best if the title is clean (for search engines and others)
• add details on lazy evaluation and referential transparency
• add example on retrying evaluation with exponential backoff
• fixed statement on the behavior of parMapN and add some extra details
• changed cancelled to canceled to be consistent with our chosen American spelling in the API 🙂
• organized IO, Fiber and Timer under the "Data Types" section
• did some configuration in build.sbt, preparing it for publishing

[alexandru]

/cc @gvolpe

[codecov-io]

Codecov Report

Merging #147 into master will decrease coverage by 5%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
- Coverage   90.03%   85.02%   -5.01%     
==========================================
  Files          47       47              
  Lines         983     1042      +59     
  Branches       79       78       -1     
==========================================
+ Hits          885      886       +1     
- Misses         98      156      +58

[gvolpe]

Awesome! :)

[LukaJCB]

👍