created rake task to import historical fx rates
optimised the country select options helper
amended fx model to accomodate only one fx date and value as per requirements
Merge branch 'master' into currency_conversion

Conflicts:
	app/helpers/application_helper.rb
	app/views/basic/sanitation/country.html.haml
	app/views/basic/water/country.html.haml
added usd and eur to rates seed
refactored report comparison helpers for better efficiency
integrated exchange rates for comparisons
rolled in refactored comparisons to advanced sanitation
updated total expenditure for years calculation
wip calculation update
amended all cost calculations for advanced water
updated calculations for affordability actual users water
finished water advanced affordability calculation updates
updated advanced water service level calculations
finished revising advanced water calculations