Fix documentation for option -w and -m of SessionBasedScript

[GoogleCodeExporter]

The current documentation says:

    -m GIGABYTES, --memory-per-core GIGABYTES 
        Set the amount of memory
        required per execution core, in gigabytes (Default:
        2). Currently, GIGABYTES can only be an integer number;
        fractional amounts are discarded.

    -w DURATION, --wall-clock-time DURATION
        Set the time limit for each job (default is ‘8’ for ‘8
        hours’). Jobs exceeding this limit will be stopped and
        considered as failed. The duration can be expressed as a whole
        number (indicating the duration in hours) or as a string in
        the form ‘hours:minutes’.

but it is currently wrong, we have to specify the correct syntax,
which comes from the `gc3libs.quantity` module, and it is in the form
"<number> <quantity>" where `<quantity>` is one of:

    'B','GB','GiB','KiB','MB','MiB','PB','PiB','TB','TiB','kB'

for memory and

    'd','day','days','h','hour','hours','hr','hrs','m','microsec','microseconds','min','mins','minute','minutes','ms','nanosec','nanoseconds','ns','s','sec','second','seconds','secs'

for duration.

Original issue reported on code.google.com by arcimbo...@gmail.com on 10 Dec 2014 at 2:16