Switch to using global context: partial fix for https://github.com/danielgtaylor/nesh/issues/4

Nesh now defaults to using the global context, which enables packages
that modify built-in prototypes to function correctly.

Takes effect for the *JavaScript* REPL, but
the default for the *CoffeeScript* REPL is of necessity kept NON-global
until a bug in CoffeeScript (as of 1.6.3) is resolved - see
https://github.com/jashkenas/coffee-script/pull/3113
Merge pull request #7 from mklement0/global-fix

Switch to using global context: partial fix for issue #4