runtime interface implementation

[jessedoyle]

After working with the Duktape API calls for some time, I've noticed that many use-cases don't require the low-level usage of many Duktape calls.

This pull request implements a higher level Duktape::Runtime class that takes inspiration from the ExecJS API.

The entire Runtime API is as follows:

• call(property, *args) - Call the property or function with the given arguments and return the result.
• call([properties], *args) - Call the property that is nested within an array of string property names.
• eval(source) - Evaluate the javascript source and return the last value.
• exec(source) - Evaluate the javascript source and always return nil.

I expect this Travis build to fail as the bug that is causing the failure was just fixed in Crystal master.

[jessedoyle]

I've tested all the specs on OS X 10.11 with the current latest Crystal master branch and everything passes. I'd really like to get a passing TravisCI build, but I think that's going to have to wait for the next release of Crystal.