ENH: Add JSON export option for DataFrame (take 2)
Second attempt for JSON pull request (original #1226) for issue #631. 

All tests pass apart from the two below, tested on 64 bit OSX and 32 bit Ubuntu. 

Time stamp JSON encoding/decoding in test_frame and test_series fails. I haven't looked into this too much, as Wes mentioned the work with timestamps was still ongoing but this appears to occur because the code always operates on the underlying numpy array rather than the pandas objects and the numpy array returns a bad Python date object to be encoded.

For numpy 1.6

``` python
(Pdb) series.index[0]  
Timestamp(2000, 1, 3, 0, 0)
(Pdb) series.index.values[0]
1970-01-11 232:00:00 
(Pdb) ujson.encode(series.index)   # encoder operates on series.index.values
'[1699200,864000,950400,1036800,1123200,1382400,1468800,1555200,1641600,1728000,1065600,1152000,1238400,1324800,1411200,1670400,1756800,921600,1008000,1094400,1353600,1440000,1526400,1612800,1699200,1036800,1123200,1209600,1296000,1382400]'

```

For numpy 1.7 the situation is improved and the encoding is correct (although the timestamps are in nanoseconds):

``` python
(Pdb) series.index[0]
Timestamp(2000, 1, 3, 0, 0)
(Pdb) series.index.values[0]
numpy.datetime64('2000-01-03T00:00:00.000000000+0000')
(Pdb) ujson.encode(series.index)
'[946857600000000000,946944000000000000,947030400000000000,947116800000000000,947203200000000000,947462400000000000,947548800000000000,947635200000000000,947721600000000000,947808000000000000,948067200000000000,948153600000000000,948240000000000000,948326400000000000,948412800000000000,948672000000000000,948758400000000000,948844800000000000,948931200000000000,949017600000000000,949276800000000000,949363200000000000,949449600000000000,949536000000000000,949622400000000000,949881600000000000,949968000000000000,950054400000000000,950140800000000000,950227200000000000]'
```
