Fix payload validator

http://developer.android.com/intl/ja/google/gcm/gcm.html
'data' field is a limit on the total size of the message (4kb)
Add registration Ids validator

http://developer.android.com/intl/ja/google/gcm/gcm.html
It must contain at least 1 and at most 1000 registration IDs.
Fix 'registration_ids' column for MySQL

https://groups.google.com/forum/#!msg/android-gcm/q2PzJTP71TY/DNX4lvDu5X4J
The regID could grow up to 4k
Merge branch 'gcm-validator'