Allow dash character in FIRApp's name. (#19)

Since Google Cloud Projects Project IDs are allowed to use a dash, they
should also be allowed in FIRApp's name.

Author: ryanwilson

Example/Core/Tests/FIRAppTest.m

@@ -20,7 +20,7 @@
 extern NSString *const kUniqueInstallFileName;
 
 NSString *const kFIRTestAppName1 = @"test_app_name_1";
-NSString *const kFIRTestAppName2 = @"test_app_name_2";
+NSString *const kFIRTestAppName2 = @"test-app-name-2";
 
 @interface FIRApp (TestInternal)
 

FirebaseCore/Core/FIRApp.m

@@ -141,9 +141,10 @@ + (void)configureWithName:(NSString *)name options:(FIROptions *)options {
     char character = [lowerCaseName characterAtIndex:charIndex];
      if (!((character >= 'a' && character <= 'z')
            || (character >= '0' && character <= '9')
-           || character == '_'))  {
+           || character == '_'
+           || character == '-'))  {
        [NSException raise:kFirebaseCoreErrorDomain format:@"App name should only contain Letters, "
-                                                      @"Numbers and Underscore."];
+                                                      @"Numbers, Underscores, and Dashes."];
      }
   }