Molecule __init__ and __repr__ respect multiplicity.

When creating a molecule via the SMILES shortcut, you can now
specify the multiplicity, and furthermore the __repr__ will 
get back what you started with:

In [1]: from rmgpy.molecule import Molecule

In [2]: print Molecule(SMILES="[CH2]").toAdjacencyList()
multiplicity 3
1 C U2 L0 E0  {2,S} {3,S}
2 H U0 L0 E0  {1,S}
3 H U0 L0 E0  {1,S}


In [3]: print Molecule(SMILES="[CH2]", multiplicity=1).toAdjacencyList()
multiplicity 1
1 C U2 L0 E0  {2,S} {3,S}
2 H U0 L0 E0  {1,S}
3 H U0 L0 E0  {1,S}


In [4]: Molecule().fromAdjacencyList(""" 1 C 2  """)
Out[4]: Molecule(SMILES="[CH2]")

In [5]: Molecule().fromAdjacencyList(""" 1 C 2S  """)
Out[5]: Molecule(SMILES="[CH2]", multiplicity=1)