Move rules fetcher in its own class

Creates a NdependRulesFetcher that fetch ndepend rules from a URL if
the corresponding settings is not empty.

If the setting stored in NdependConfig.NDEPEND_RULES_URL_KEY is empty
then retrieve the default rules.xml file included in resources.

Change-Id: I4d54a046adc231c9724e3823132d5bf625e51893