Partial indexing only valid for ordered time series
This was mentioned on [StackOverflow](http://stackoverflow.com/questions/13737992/indexing-timeseries-by-date-string), I thought I ought to post it here. I'm not sure whether or not this is a bug:

You can select by a date string in an ordered tseries, but not in an ordered one:

```
import pandas as pd
from numpy.random import randn
from random import shuffle
rng = pd.date_range(start='2011-01-01', end='2011-12-31')
rng2 = list(rng)
shuffle(rng2)
```

```
ts = pd.Series(randn(len(rng)), index=rng)
ts2 = pd.Series(randn(len(rng)), index=rng2)
```

```
ts.index
<class 'pandas.tseries.index.DatetimeIndex'>
[2011-01-01 00:00:00, ..., 2011-12-31 00:00:00]
Length: 365, Freq: D, Timezone: None

ts['2011-01-01']
# -1.1454418070543406
```

```
ts2.index
<class 'pandas.tseries.index.DatetimeIndex'>
[2011-04-16 00:00:00, ..., 2011-03-10 00:00:00]
Length: 365, Freq: None, Timezone: None

ts2['2011-01-01'] # same for ts2.ix['2011-01-01']
```

```
TimeSeriesError                           Traceback (most recent call last)
<ipython-input-112-051e81424a0d> in <module>()
----> 1 ts2['2011-01-01']

/Library/Python/2.7/site-packages/pandas-0.9.0-py2.7-macosx-10.7-intel.egg/pandas/core/series.pyc in __getitem__(self, key)
    462     def __getitem__(self, key):
    463         try:
--> 464             return self.index.get_value(self, key)
    465         except InvalidIndexError:
    466             pass

/Library/Python/2.7/site-packages/pandas-0.9.0-py2.7-macosx-10.7-intel.egg/pandas/tseries/index.pyc in get_value(self, series, key)
   1015 
   1016             try:
-> 1017                 loc = self._get_string_slice(key)
   1018                 return series[loc]
   1019             except (TypeError, ValueError, KeyError):

/Library/Python/2.7/site-packages/pandas-0.9.0-py2.7-macosx-10.7-intel.egg/pandas/tseries/index.pyc in _get_string_slice(self, key)
   1062         asdt, parsed, reso = parse_time_string(key, freq)
   1063         key = asdt
-> 1064         loc = self._partial_date_slice(reso, parsed)
   1065         return loc
   1066 

/Library/Python/2.7/site-packages/pandas-0.9.0-py2.7-macosx-10.7-intel.egg/pandas/tseries/index.pyc in _partial_date_slice(self, reso, parsed)
    977     def _partial_date_slice(self, reso, parsed):
    978         if not self.is_monotonic:
--> 979             raise TimeSeriesError('Partial indexing only valid for ordered time'
    980                                   ' series')
    981 

TimeSeriesError: Partial indexing only valid for ordered time series
```
