Switch to using global context: partial fix for https://github.com/danie...

...lgtaylor/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

Note:

* I've made no attempt to indicate to the user that - for now - the global context is NOT active when running the _CoffeeScript_ REPL - wonder if it makes sense to issue a warning on startup.
* The `semver` dependency added is not actively used yet, but will come in handy for version comparison once the specific CoffeeScript version with the required fix is known.