improve serialization usage

[asmodehn]

It seems some people are having trouble grasping how serialization between JSON and ROS work.

It is currently too much "magic"

It would help to expose tools to th use that directly do the conversion.

Also some (tested) doc about it wouldn't hurt, people don't usually look into test code :

>>> import pyros.rosinterface.message_conversion as msg_conv
>>> test_data = { 'data': 'testing' }
>>> import std_msgs.msg as std_msgs
>>> rosstr = std_msgs.String()
>>> rosstr
data: ''
>>> rosstr._type
'std_msgs/String'
>>> msg_conv.populate_instance( test_data, rosstr)
data: testing
>>> msg_conv.extract_values(rosstr)
{'data': 'testing'}

[asmodehn]

This is actually a good reason to have a separated library for ROS <-> python message conversion...

[asmodehn]

Also : https://www.youtube.com/watch?v=7cC3_jGwl_U