After a weekend of meetings in Washington and Paris, finance ministers from leading industrialized nations have pledged to work together to tackle the financial crisis.   The G7 nations (the US, Japan, Britain, Germany, France, Italy and Canada) issued a five-point plan of "decisive action" to unfreeze credit markets, after their first meeting in Washington Saturday.    The plan is intended to protect major banks and financial institutions from failure and ensure they can raise capital from public and private sources.