Added ComputationalSimulations directory, with first simulation directory: RegisterMachine.

Will eventually contain a system to simulate Register Machines (http://en.wikipedia.org/wiki/Register_machine).

This commit adds a module encoder.py, providing a bijectional function to encode a pair of natural numbers as a single natural number.
Added checking to encode_pair_fat tn ensure that input sequence only contains natural numbers.
Changed name of encode_pair_fat to encode_pair to properly reflect functionality.
Fixed incorrect type of fat parameter in encode_pair.
Added encode_list to encoder.py.

encode_list provides a bijectional function to represent lists of natural numbers as natural numbers.
Added tests for RegisterMachine ComputationalSimulation: ComputationalSimulation/RegisterMachine/tests.py
Added encode_program_instruction() to encoder.py to encode a list of program_instructions to a single number.
Added tests for encode_program_instructions.
FIxed bugs in encode_program_instructions. Also slightly changed instruction format.
Added a test to check that encode_program_instructions correctly detects incorrectly formatted instructions.