Release 0.21.0

Concuerror 0.20.0

Concuerror 0.19

Update CHANGELOG

Add Homepage link to pages too

[ci skip]

Merge pull request #109 from aronisstav/testing

Bounding and usage improvements

Merge pull request #105 from aronisstav/testing

Fix behaviour of demonitor

Merge pull request #94 from aronisstav/prepare19

Changes for Erlang/OTP 19

Files for EUC 2015 tutorial