Add a nickname column to database.txt

This pull request adds an extra column to database.txt consisting of an informal molecule nickname.  The specification of the nickname is (almost by definition) not strict.  I hoped to achieve a compromise between the following desired but conflicting properties:

1) Unique (i.e. minimize the number of duplicates).
2) Short - the issue with IUPAC names is that some of them became too unwieldy to print out in ForceBalance.
3) Specific - we should get results for this molecule by Googling its nickname.

I use the nickname in ForceBalance simply as a display field.  I also display the mobley_ID as well.  The ForceBalance printout is a fixed-width table which is easy to read (in my opinion):

```
#==============================================================================================================================================#
#|                                                     Hydration free energies (kcal/mol)                                                     |#
#|                                     Target: FreeSolv  Type: Hydration_OpenMM  Objective = 1.63924e+03                                      |#
#|  ID                                                Nickname   Reference +- StdErr   Calculated +- StdErr   Calc-Ref     Weight   Residual  |#
#==============================================================================================================================================#
    mobley_1017962                            methyl_hexanoate      -2.490 +-  0.600       -3.778 +-  0.011     -1.288      0.000      0.000
    mobley_1019269                                  butan-1-ol      -4.720 +-  0.600       -3.272 +-  0.016      1.448      1.000      2.096
    mobley_1034539            2,3,3',4,4',5-hexachlorobiphenyl      -3.040 +-  0.100        1.271 +-  0.005      4.311      0.000      0.000
    mobley_1036761                             cyclohexanamine      -4.590 +-  0.600       -3.091 +-  0.029      1.499      0.000      0.000
    mobley_1046331                              phenyl_formate      -3.820 +-  0.600       -7.897 +-  0.018     -4.077      1.000     16.619
    mobley_1075836                           methyl_propanoate      -2.930 +-  0.600       -4.841 +-  0.009     -1.911      1.000      3.651
    mobley_1079207                         1,3-dichlorobenzene      -0.980 +-  0.600       -0.203 +-  0.002      0.777      0.000      0.000
    mobley_1107178                                  iodoethane      -0.740 +-  0.600       -2.617 +-  0.004     -1.877      1.000      3.522
    mobley_1139153                      2,2,4-trimethylpentane       2.890 +-  0.600        3.055 +-  0.003      0.165      1.000      0.027
```

Thanks,

- Lee-Ping