Export DataFrame with objects to Excel creates error.
pandas version 0.17.0

I'm using a Class Name to set names.

```
from pandas import DataFrame

class Name(object):
    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return self.name
```

I want to create a simple DataFrame

```
df = DataFrame([{"Hi":Name("hello"), "My":"Friend"}])
# df = DataFrame([{Name("Hi"):"hello", "My":"Friend"}])
```

I try to export it in Excel with:

```
from pandas import ExcelWriter
xlw = ExcelWriter("welcome.xlsx")
df.to_excel(xlw, "test")
xlw.save()
```

but I have the following error:

[![error message trying to export df to excel](http://i.stack.imgur.com/v2hqX.png)](http://i.stack.imgur.com/v2hqX.png)

in the pandas/io/excel.py

```
def _conv_value(val):
# Convert numpy types to Python types for the Excel writers.
if com.is_integer(val):
    val = int(val)
elif com.is_float(val):
    val = float(val)
elif com.is_bool(val):
    val = bool(val)
elif isinstance(val, Period):
    val = "%s" % val
elif com.is_list_like(val):
    val = str(val)
# <---- a test is missing here.
return val
```

adding else:
     val = str(val)   solve the pb.

the export is correct with 
- df =  DataFrame([{"Hi":"hello", "My":"Friend"}])
