Feature/freshbooks invoice

I added another script toggl-freshbooks-invoice.rb which generates a FreshBooks invoice from Toggl time entries for a specified date range and client.

So toggl-freshbooks.rb generates time entries in FreshBooks whereas toggl-freshbooks-invoice.rb generates an  invoice in FreshBooks but not FreshBooks time entries.

I extracted some common functionality to includes in the lib directory. The error handling for the getTogglSummary function I extracted is a bit more verbose than the original code, which I hope won't be an issue.

See #5