Moved config to plist + isLocalTwitterAccountAvailable

ended up doing some of it myself :-)

* adding config from plist
* added config to plist + added isLocalTwitterAccountAvailable

a few issue that I encountered, which I think that to should mentioned in the docs:

add URL Schemes for: af-twitter
and add it in a const:
``` objective-c
// Register your application to launch from a custom URL scheme, and use that with the path /success as your callback URL.
// see https://github.com/AFNetworking/AFOAuth1Client
#define TWITTER_LOGIN_URL_PREFIX @"af-twitter"
```
(worth mentioning that you have to specify some 'valid' URL in your Twitter Application Details - "Callback URL". and this url is only used for iOS redirect)
 

also, I would add the Twitter API key/secret to the plist and mention it in the docs.
much easier then finding it in:
https://github.com/seivan/SHAccountManager/blob/master/SHAccountManagerExample/SHApplication.h