Refactor common functionality in the TwoSided factors to AbstractTwoSidedFactor.java
Merge remote-tracking branch 'marc/master'

Conflicts:
	src/es/csic/iiia/maxsum/factors/AbstractTwoSidedFactor.java
	src/es/csic/iiia/maxsum/factors/TwoSidedEqualityFactor.java
	src/es/csic/iiia/maxsum/factors/TwoSidedReserveFactor.java
Empty buffer after sending messages in TickCommunicationAdapter
The value for the last message receive from a neighbor is 0 if there is no previous message from that neighbor.
Merge remote-tracking branch 'marc/master'
Merge pull request #4 from tonipenya/master

Tick communicator clears buffer after sending messages. AbstractFactor.getMessage returns 0 if there is no message for that neighbor.