Support defining multiple instances by specifying instanceName in getInstance()

[zihejia]

addresses #703. This will add the following APIs to MixpanelAPI:

/**
...
 * @param instanceName The name you want to uniquely identify the Mixpanel Instance.
   It is useful when you want more than one Mixpanel instance under the same project token.
...
**/

getInstance(Context context, String token, String instanceName)
getInstance(Context context, String token, boolean optOutTrackingDefault, String instanceName)
getInstance(Context context, String token, JSONObject superProperties, String instanceName)
getInstance(Context context, String token, boolean optOutTrackingDefault, JSONObject superProperties, String instanceName)

Please note: If you are going to add instanceName to getInstance on your existing implementation. getInstance will start using instanceName as the instance identifier rather than token, so you might lose some of the stored properties including the distinct Id under the token. We'd recommend using it when you need to create more than one instance under the same project token. You won't lose any events and user profile updates.

[jaredmixpanel]

LGTM