Remove isomorphic-fetch dependency

This refactors the package not to use the isomorphic-fetch package as this pollutes the global scope. This change avoids there being multiple versions of fetch being used in an application.

A global fetch instance will still be used if available (such as in apps which use n-express), otherwise it will fall back to its own node-fetch implementation. This is the same behaviour as implemented by the n-fetch package.

#104