DatetimeIndex::inferred_freq is misleading
Looking at these three methods on DatetimeIndex:

``` python
    # alias to offset
    @property
    def freq(self):
        return self.offset

    @cache_readonly
    def inferred_freq(self):
        try:
            return infer_freq(self)
        except ValueError:
            return None

    @property
    def freqstr(self):
        return self.offset.freqstr
```

you would think that `freqstr` returns a string and `inferred_freq` and `freq` return "frequencies" (i.e. offsets). In reality `inferred_freq` is a string as well.

I am not sure of the best way to deal with this issue. At the very least I would suggest adding a `inferred_freq_offset` property. Perhaps also deprecate `inferred_freq` and create a `inferred_freqstr`.

Tracing this method, you find `_FrequencyInferer::get_freq` which returns a string rather than an Offset.
