ENH: Default for Excel sheetname
Excel workbooks have 'Sheet1' as the default sheet. Can `pd.read_excel` have this as a default? `pandas.io.excel.read_excel(io, sheetname='Sheet1', **kwds)`

This makes pandas more accessible to the user transitioning from Excel by avoiding the common error below:

```
In [9]: test = pd.read_excel('/Users/clark/Desktop/test.xlsx')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-9-14880f6f18e4> in <module>()
----> 1 test = pd.read_excel('/Users/clark/Desktop/test.xlsx')

TypeError: read_excel() takes exactly 2 arguments (1 given)
```
