Make compatible with xctool and Xcode 7

[ExtremeMan]

Current implementation isn't compatible with xctool and Xcode 7 because xctool couldn't query test cases.
This PR fixes it.

Test plan:

xctool -project GoogleTestExample.xcodeproj/ -scheme demo -sdk iphonesimulator test

# before these changes
[Info] Collecting info for testables... (889 ms)
run-test TestBundle.xctest (iphonesimulator9.0, iPhone 4s, logic-test)
  ✓ -[ExampleObjCTest testExample] (3 ms)
  1 passed, 0 failed, 0 errored, 1 total (3 ms)

# after these changes
[Info] Collecting info for testables... (855 ms)
run-test TestBundle.xctest (iphonesimulator9.0, iPhone 4s, logic-test)
  ✓ -[ExampleObjCTest testExample] (4 ms)
  ✓ -[Counter Increment] (0 ms)
  ✓ -[Counter InitialState] (0 ms)
  3 passed, 0 failed, 0 errored, 3 total (5 ms)

[ExtremeMan]

Note that this was done because registerTestClasses could implicitly invoke [GoogleTestCase initialize] and cause duplicated class registration because check if ([GoogleTestFilterMap count]) will fail.

[darknoon]

Calling +initialize is frowned upon in ObjC; I think what you want is

if (self == [GoogleTestCase class]) {
    [self registerTestClasses];     
}

Does that solve it getting called twice?

[OlivierSohn]

Hello,

I had the bug where registerTestClasses was called twice, this is how I fixed it (I created the pull request also):

    NSArray* loadedClasses = [notification.userInfo objectForKey:NSLoadedClasses];

    if( loadedClasses != nil ) {
        if( [loadedClasses indexOfObject:@"GoogleTestLoader"] != NSNotFound) {
            [self registerTestClasses];
        }
    }

[ExtremeMan]

Ping @mattstevens.