Fixes #15: Slimerjs freezed on JS errors

It appears when an error occured during webpage.evaluate().
A default error handler is used now on webpage.onError.
Adds an internal API to retrieve the webpage object from a window object
WIP on issue #41: implementation of onNavigationRequested

It needs some additionnal work...
Closes #41: support of webpage.navigationLocked and onNavigationRequested

There is still an issue because of the limitation of the Mozilla API:
we cannot determinate the navigation type, so onNavigationRequested
does always receive "Undefined".
Fixes bug: webpage.settings was in read only mode

refs #43
Implements require.extensions

It allows to declare loaders for specific filename extensions.
It is needed for the support of CoffeeScript issue #36
Issue #36: Adds dummy modules needed by CoffeeScript
issue #36: Exit code of mozilla should be used as slimerjs exit code
Something was missing for the implem of require.extensions...
Issue #36: support of Coffee-Script modules

We embed the same version of Coffee-Script that is embedded into PhantomJS 1.9
Closes #36: support of scripts written with Coffee-Script

This change allow to give directly a coffee-script script to slimerjs
Support of JSON modules