Updated the requirements.txt file for PIP

Only real change is requiring cython>=0.19
(0.16 works, but 0.17 and 0.18 don't)
Oh, and I added argparse and gprof2dot, for the profiling post-processing.
Besides that it's mostly rearranging and adding comments.
Fixed bug in reading of G3 energies from Gaussian log files.

Apparently the line to search for contains "G3(0 K)", not "G3 (0 K)". That's
what I get for assuming Gaussian is consistent I guess, since the equivalent
line for CBS-QB3 is "CBS-QB3 (0 K)'.
Fixed bug in Network.solveFullME() and Network.solveReducedME().

The total concentration was being calculated in cm^3/mol assuming the
pressure was in bar, since it was originally written when these were true.
However, RMG-Py uses SI units for things, however, so pressures are in Pa
and total concentration should be in m^3/mol. (These two effects very nearly
cancel, but not quite.)
Better error message when unable to determine atom type.

The new error message will now also show the number of each type of bond
that it identified, which is the key means of determining the atom type
besides the element.
Fixed another bug in reading of G3 energies from Gaussian log files.

"0 K", not "O K"