Remove donations from README
Update integration notes
Correct YARD docs
Lift parallel into runtime dependency
Use parallel gem for isolation and signalling

The code looks solid and it does not make too much sense to duplicate
the efford into a better version when I would mostly change the code
style, not the effective sequence / composition of syscalls.
Do not use an evil example for an isolated global side effect
Adjust flay score
Prepare version 0.5.23
Allow failures on 2.1.2
Remove unneded block args
Compact warnings emmited on namespace matching
Remove useless comment and fix yard docs
Move CLI::Builder into own file
Move complex matcher building out of CLI responsibilities
Move cache instide match builder
Perform test matching on Mutant::Expression instances
Fix YARD docs
Use next to skip iteration
Fix test selection scope expansion

Restores the pre expression based test selection since the change was
unintentional made.
Add singleton and instance methods expression