Adds swift example. EXPR might not be 100% valid

[ajm188]

If I really cared, I would substitute EXPR with what is a valid expression
according to the swift grammar, but I think this conveys the general idea.

Address #26.

Examples:

(venv)~/github/undebt (add-swift-example) $ echo "if let x = a where x == y {" | undebt -p undebt/examples/swift.py
if let x = a, x == y {
(venv)~/github/undebt (add-swift-example) $ echo "if let x = a, y = b, z = c where x == y && y != z {" | undebt -p undebt/examples/swift.py
if let x = a, let y = b, let z = c, x == y && y != z {
(venv)~/github/undebt (add-swift-example) $ echo "if let x = a + 1 where x == y {" | undebt -p undebt/examples/swift.py
if let x = a + 1, x == y {

[coveralls]

Coverage decreased (-2.6%) to 94.19% when pulling 7b39e29 on ajm188:add-swift-example into c5c97fd on Yelp:master.

[ajm188]

Well, I guess I should add those examples as tests, huh? 😛

[coveralls]

Coverage increased (+0.1%) to 96.888% when pulling 1b2771a on ajm188:add-swift-example into c5c97fd on Yelp:master.

[evhub]

@ajm188 Looking good! Do you think it would be a good idea to add this as another entry on the examples documentation page?

[ajm188]

@evhub yeah, since it will help answer the general "does this work for my language" questions :) i'll get right on it

[evhub]

@ajm188 LGTM!

[coveralls]

Coverage increased (+0.1%) to 96.888% when pulling 1cad3dd on ajm188:add-swift-example into c5c97fd on Yelp:master.

[coveralls]

Coverage increased (+0.1%) to 96.888% when pulling 1cad3dd on ajm188:add-swift-example into c5c97fd on Yelp:master.