Sim#return: Update the TOSU:TOSH:TOSL registers.

This is essential because the values of those registers are used by the
simulator when simulating a return.
Sim#stack_push: Update TOSU:TOSH:TOSL registers.
rubocop: Added blank lines around access modifiers in the code.
rubocop: Remove empty lines around body.
rubocop: Use 'until' instead of negated while.
rubocop: Use Ruby 1.9 hash syntax.
rubocop: Use \ to concatenate strings through newlines.
rubocop: Don't compare to nil.
rubocop: Put underscores in integers.
rubocop: Use 'proc' instead 'Proc.new' and have space after commas.
rubocop: Don't shadow variables and don't have one-line methods.
rubocop: Add a lot of spaces.
rubocop: Add a lot of spaces.

These changes were supposed to be part of the last commit.
Switch back to using the official yard gem.
Added the XC8 variable issue to KnownIssues.md.
Bumped version to 0.2.5.  Updated change log.