face: Multiple matching for setInterestFilter and expressInterest callbacks

As of this commit, all callbacks registered by setInterestFilter will be
called for every interest that is matching the filter.

Similarly for expressed Interest, all pending Interest will be satisfied
by any incoming matching Data.

Change-Id: I2463bd4114a2a1cdbe92ff03805adc5c404266a9
util: Merge helpers folder into util.

Change-Id: Ie8afba2cfe55c4a7587f3a3f39c120dbabb7a5ca
util: Add helper method to load/save object from/to file.

Change-Id: I7417bbf85865e15c02e5dec170265bcdd3f2bae4
security: Add prepareUnsignedIdentityCertificate method in KeyChain

Change-Id: Iee553db9aa050a3e02e59917ee72e3cb3a84d22e