Add flattr button to readme
Set ruby version on circle to 2.1.4
Add a README section about the MRI stuck/crash behavior
Handle :rational type nodes via generic mutator
Correct YARD docs
Bump unparser dependency
Whitespace
Bump unparser and parser dependency
Handle literal complex nodes via generic mutator
Fix expected AST differencies from unparser changes
Add __FILE__ and __LINE__ to nodes handled by generic mutator
Use actor based parallelization
Add Subject#source_lines
Add Mutant::Tracer

* Depends on `TracePoint` thus removes 1.9 compatiblity. Do not expect
  someone is harmed by this.