Whitespace
Avoid string mutation
Avoid use of mutable strings in ExceptionState
Avoid using mutable string methods
Remove String heritage from IOStub

Now it’s a plain Ruby class.
Prefer block/closure style in #class_eval

Opal doesn’t support class_eval with String (due to limited control
given by JavaScript `eval()` function) but works well with a block.
Add Opal as a target implementation
Calculate current MSpec lib path relative to current path

Instead of using an absolute path local to *my* machine.
Merge pull request #55 from elia/opal-avoid-string-class-eval

Prefer block/closure style in #class_eval
Cleanup the default command for `-t opal`
Merge pull request #57 from elia/opal-avoid-mutable-strings

Opal avoid mutable strings
Merge pull request #58 from elia/opal--t-option

Add Opal as a target implementation
Add some guards to skip stuff unsupported by Opal

Most of it is wrapping backticks that in Opal are used to insert
JavaScript code directly.