Fixes bugs with getTimeline

This commit addresses the following bugs with getTimeline:

1) The count parameter doesn't match the documentation for Twitter's API. If an optionalCount is specified, getTimeline should return at most that number of statuses. However, the existing implementation can return up to (4 * optionalCount) statuses instead. (eg. If you request a user's timeline with a count of 10 and that user has 50 posts, the method will return 40 posts.) Therefore, I've removed the repeating functionality inside of getTimeline.
2) The "mparams" dictionary leaks memory if the getResource:successBlock:errorBlock: call encounters an error.
3) There are no checks for null blocks (successBlock and errorBlock)