Implement pangram

Note: This won't pass CI. I haven't yet put the problem into
config.json. Waiting on exercism#127

Implements the Pangram problem and follows the current canonical test
suite.

exercises/pangram/Cargo.toml

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+[package]
+name = "pangram"
+version = "0.0.0"

exercises/pangram/example.rs

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+use std::collections::BTreeSet;
+use std::ascii::AsciiExt;
+use std::iter::FromIterator;
+
+pub fn is_pangram(sentence: &str) -> bool {
+    sentence.to_lowercase()
+            .chars()
+            .filter(|c| c.is_alphabetic())
+            .filter(|c| c.is_ascii())
+            .collect::<BTreeSet<char>>() == english_letter_set()
+
+}
+
+fn english_letter_set() -> BTreeSet<char> {
+    BTreeSet::from_iter(ENGLISH_ALPHABET.chars())
+}
+
+const ENGLISH_ALPHABET: &'static str = "abcdefghijklmnopqrstuvwxyz";

exercises/pangram/tests/pangram.rs

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+extern crate pangram;
+
+use pangram::*;
+
+#[test]
+fn empty_strings_are_not_pangrams() {
+    let sentence = "";
+    assert!(!is_pangram(&sentence));
+}
+
+#[test]
+#[ignore]
+fn classic_pangram_is_a_pangram() {
+    let sentence = "the quick brown fox jumps over the lazy dog";
+    assert!(is_pangram(&sentence));
+}
+
+#[test]
+#[ignore]
+fn pangrams_must_have_all_letters() {
+    let sentence = "a quick movement of the enemy will jeopardize five gunboats";
+    assert!(!is_pangram(&sentence));
+}
+
+#[test]
+#[ignore]
+fn pangrams_must_have_all_letters_two() {
+    let sentence = "the quick brown fish jumps over the lazy dog";
+    assert!(!is_pangram(&sentence));
+}
+
+#[test]
+#[ignore]
+fn underscores_do_not_affect_pangrams() {
+    let sentence = "the_quick_brown_fox_jumps_over_the_lazy_dog";
+    assert!(is_pangram(&sentence));
+}
+
+#[test]
+#[ignore]
+fn numbers_do_not_affect_pangrams() {
+    let sentence = "the 1 quick brown fox jumps over the 2 lazy dogs";
+    assert!(is_pangram(&sentence));
+}
+
+#[test]
+#[ignore]
+fn numbers_can_not_replace_numbers() {
+    let sentence = "7h3 qu1ck brown fox jumps ov3r 7h3 lazy dog";
+    assert!(!is_pangram(&sentence));
+}
+
+#[test]
+#[ignore]
+fn capitals_and_punctuation_can_be_in_pangrams() {
+    let sentence = "\"Five quacking Zephyrs jolt my wax bed.\"";
+    assert!(is_pangram(&sentence));
+}
+
+#[test]
+#[ignore]
+fn non_ascii_characters_can_be_in_pangrams() {
+    let sentence = "Victor jagt zwölf Boxkämpfer quer über den großen Sylter Deich.";
+    assert!(is_pangram(&sentence));
+}