Expose TXSourceString properties

Exposes the properties of `TXSourceString` making them public readonly
so that other applications (like the Transifex CLI tool) can use them
either by querying them or by extending the `TXSourceString` class.

Ref: https://trello.com/c/1zQZUJ8E/48-cli-list-found-strings-on-push-command

Sources/Transifex/SourceString.swift

@@ -14,20 +14,20 @@ import Foundation
 /// properties of the struct while also being able to be used both by Swift and Objective-C applications.
 public final class TXSourceString: NSObject {
     /// The key that was generated by the `txGenerateKey()` method
-    let key: String
+    public let key: String
     /// The source string
-    let sourceString: String
+    public let sourceString: String
     /// An optional developer comment
-    let developerComment: String?
+    public let developerComment: String?
     /// A list of relative file paths where this string is located.
-    let occurrences: [String]
+    public let occurrences: [String]
     /// A list of tags accompanying this source string
-    let tags: [String]?
+    public let tags: [String]?
     /// A limit provided by the developer that should be respected by translators when translating the
     /// source string, 0 means no limit.
-    let characterLimit: Int
+    public let characterLimit: Int
     /// A list of strings providing more context to the source string
-    let context: [String]?
+    public let context: [String]?
 
     /// Public and designated constructor.
     ///