Period() docstring inconsistent with code when freq has a mult != 1
(I love Pandas!  Great work!  One small issue...)

The docstring for the `Period` constructor reads:

```
freq : str, default None
    e.g., 'B' for businessday, ('T', 5) or '5T' for 5 minutes
```

i.e. the docstring says that `freq` can be multiples of a period code (e.g. 5 minutes).

But if we try to use `mult != 1` then we get an exception:

``` Python
In [43]: pd.Period('2013-01-01', freq='5T')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-43-770fd8956b5c> in <module>()
----> 1 pd.Period('2013-01-01', freq='5T')

/usr/local/lib/python2.7/dist-packages/pandas/tseries/period.pyc in __init__(self, value, freq, ordinal, year, month, quarter, day, hour, minute, second)
    120         base, mult = _gfc(freq)
    121         if mult != 1:
--> 122             raise ValueError('Only mult == 1 supported')
    123 
    124         if self.ordinal is None:

ValueError: Only mult == 1 supported
```

This same error is given if we provide `freq` as a tuple like `freq=('T', 5)`

So either the code can't handle `mult != 1` (in which case the docstring is wrong). Or, if the docstring is _correct_ (the code _can_ handle a `mult != 1`) then the exception is wrong.

(For my own work, I would love to be able to use arbitrary-length `Period` instances.  Here's a related StackOverflow question that I have just asked: [Irregular, non-contiguous Periods in Pandas](http://stackoverflow.com/questions/18486106/irregular-non-contiguous-periods-in-pandas))

(I'm using Pandas 0.12)
