BUG: Panel.from_dict does not set dtype when specified
currently `Panel.from_dict` is just ignoring the `dtype` argument:

```
In [1]: import pandas as pd
In [2]: import numpy as np
In [3]: df = pd.DataFrame(np.array(range(6)).reshape(3, 2))
In [4]: pa = pd.Panel.from_dict({'a': df, 'b': df}, dtype=float)
In [5]: pa.dtypes
Out[5]:
a    int64
b    int64
dtype: object
```

this PR sets the `dtype` and fixes the docstrings in some places 
