Added the packer methods

[rtu-dataframe]

Added the packer methods to keep simple the data reading and writing, as requested in the issue #61 , i've created three functions in the utils.py in order to simplify the data packing and unpacking while reading/writing some different data types like Float, Double, etc... (Data Types Here)

In everyday use, you can simply use those methods importing the utils.py module and read (for example) a float value like shown below:

f = data_unpacker(response, '>', 'f')

instead of:

f = struct.unpack('>f', struct.pack('>H', high_byte) + struct.pack('>H', low_byte)[0]

[coveralls]

Coverage decreased (-1.3%) to 95.039% when pulling fd08bd9 on Simonefardella:master into 0560a42 on AdvancedClimateSystems:master.

[coveralls]

Coverage decreased (-0.4%) to 95.921% when pulling dc1d82c on Simonefardella:master into 0560a42 on AdvancedClimateSystems:master.

[rtu-dataframe]

A new pull request will be submitted after some fix