Offer buffer-oriented API for I/O (#25)

https://github.com/stratosphere/stratosphere/issues/25

The runtime offered a record-oriented API for data transfers, which
* resulted in unnecessary data (de)serialization,
* complicated the upcoming fault tolerance implementation, and
* blocked more efficient implementations of higher-level operators.

With this commit, the runtime offers a buffer-oriented API for the
output side (sending), which is oblivious to records. The buffer
oriented input side (receiving) is still to be implemented.