Changed the display of timestamps

Execution times are now shown on the memory graph using a kind of
brackets instead of just vertical lines. This makes reading the graph
easier.

Graphing is now correct for any sampling frequency.
Added --interval option to mprofile

This options is used to change the sampling period. Example:
mprofile -T 0.2 <program>

Will execute <program> and record its memory usage every .2s.