Changed format of key for storing entries in some kinetics databases.

Previously the index of each entry was being used as the key. In principle
this should work fine, as each entry should have a unique index. In
practice, however, there are currently many entries in the database
(especially for H_Abstraction) that share an index, mainly because of
the sulfur rules. This commit enables the ability to handle multiple
entries with the same index (as long as they have different labels).
Molecule.getCp0() now properly handles monatomics.

Monatomics have no rotational degrees of freedom, so they have a Cp0 of
2.5R.
Added first version of tool for merging RMG-generated models.

Like many of the existing web tools, you need the Chemkin file and the
species dictionary for each model you are merging. The models to be
merged should be given in order of priority, since, when common species
and reactions are encountered, the earlier model is given priority.
You can also merge more than two models at once if you are so inclined.

The standalone script mergeModels.py could use a few more options (e.g.
the ability to specify where the output files are written), but it is
usable. The actual merging is done in the ReactionModel class, so it
should be easy to set this up as a web tool as well.
Update documentation/source/users/rmg/installation/windows.rst

Added a link to get the python27.def file on Windows in case pexports doesn't work.

On some systems (eg Windows 7 64 bit) the pexports method of generating the python27.def file 
doesn't seem to work, but apparently you can just download the file.
Print warning comments when thermo or kinetics differ during model merging.