Extract 'remember' feature from substeps-webdriver

[davemoss]

Remembered data can't be used cross-module.

e.g. it's currently not possible to identify text on a web page in substeps-webdriver, and then use the same value in subseps-email without creating a code dependency between these modules.

[beercanx]

That's what I thought com.technophobia.substeps.runner.ExecutionContext in substeps-api was for.

    public static void put(final Scope scope, final String key, final Object value);
    public static Object get(final Scope scope, final String key);