Added token initialization functions and ability to limit number of tokens to be entered

Hey Tom,

Great library, Just started using as part of a project at work for our app that we're releasing soon.

Added functions for initializing the token field with tokens:
(void)addTokensWithTitleList:(NSString *)titleList; 
(void)addTokensWithTitleArray:(NSArray *)titleArray;

The first one takes a comma separated list string list.


Also added a tokenLimit to TokenField and a control statement so that you can limit how many tokens a user is allowed to enter. The value is defaulted to -1 for unlimited tokens and set like this:
[_tokenFieldView.tokenField setTokenLimit:5];