Graph plotting

This is an enhancement to give memory-profiler the ability to record global memory consumption along time, store it on disk and plot it afterward.

Three commands have been added, plus some small modifications to the memory_profiler module: mprofile, for launching a process, mplot for plotting the result, and maction for manipulating the generated profile files.

There is no documentation yet, apart for the ouput of -h options. A simple example:

    mprofile <executable>
    mplot

There is a special mode for python programs: any function decorated with @profile will be timestamped when entered and left, and special markers will be plotted on the memory usage graph. Example:

    cd memory_profiler/test
    mprofile --python test_mprofile.py
    mplot

Feel free to comment and criticize, some parts of the code don't satisfy me completely.