Added --title option to 'mprof plot'

This option allows to set the title of figures created by 'mprof plot'.
Got rid of numpy is read_mprofile_file

Numpy is still required for plotting, but only when pylab is used.
Proper handling of empty profile files

When interrupting "mprof run", an empty file can be generated, which
caused problem with "mprof plot". A clear error message is now displayed
to the user.
Command line is saved in profile file

The full command line of what was run during the profiling is now
saved into the profile file. It is used to set the graph title with
'mprof plot'.
Plot all memory usage information

Memory usage when entering and leaving a function is now included
with the other measurements. This fixes some discrepancies between
brackets and curves when sampling frequency was low.

Add an option to prevent displaying of timestamps in plots
(mprof plot -n)
Merge pull request #57 from pgervais/plot_options

Plot improvements