Added os module, with "executeCommand" function.

This function will be use everywhere we need to execute an os command.
It's more flexible, if we want to replace the "subprocess.Popen" call in the future for example.
Returns the result string.
Improved os.executeCommand function.

executeCommand function catch now correctly pipes and vars with double quotes.
We use "shlex.split" in POSIX mode, this python function returns the command formatted for subprocess module.
Spelling fixes in os module
Renamed "os" module to "system"

Big mistake: "os" is already a python core module. We cannot use this name, or our scripts will not able to find the native module.
In collectors, use new executeCommand function instead of subprocess calls

It's more readable and convenient, no? :)
Merge pull request #27 from miniche/feature-os-class

Added os module, and executeCommand function