Add Haskell solution for Problem 19

Haskell/p019.hs

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+-- You are given the following information, but you may prefer to do some research for yourself.
+--
+-- 1 Jan 1900 was a Monday.
+-- Thirty days has September,
+-- April, June and November.
+-- All the rest have thirty-one,
+-- Saving February alone,
+-- Which has twenty-eight, rain or shine.
+-- And on leap years, twenty-nine.
+-- A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.
+--
+-- How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?
+
+import Data.Time.Calendar
+
+countSundaysFirst :: Day -> Day -> Int
+countSundaysFirst start end = let days = [start .. end]
+                              in length $ filter (\x -> dayOfWeek x == Sunday && [last (init (show x)), last (show x)] == "01") days
+
+main = do
+    let startDate = fromGregorian 1901 1 1
+        endDate = fromGregorian 2000 12 31
+        result = countSundaysFirst startDate endDate
+    putStrLn $ "Project Euler, Problem 19\n"
+        ++ "Answer: " ++ show result