Adding units specification to mem_limit variable

This PR enables the user to create a memory limited container in a human readable form:

    container = c.create_container(image, 'sleep 12', mem_limit='128m')

Instead of `mem_limit` parameter accepting float values only, now the user can define a string with a units character (similar to the way the docker command line works)

A detailed discussion about this feature can be found here: #252