BLD: windows builds failing in pandas.json
cc @Komnomnomnom

was reported by Gagi, but I reproduced this on building on AMD64

This is current master

can you take a look?

thanks

```
building 'pandas.json' extension
creating build\temp.win-amd64-2.7\Release\pandas\src\ujson
creating build\temp.win-amd64-2.7\Release\pandas\src\ujson\python
creating build\temp.win-amd64-2.7\Release\pandas\src\ujson\lib
C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\Bin\amd64\cl.exe /c /nologo /Ox /MD /W3 /GS- /DNDEBUG -Ipandas/src/ujson/python -Ipandas/src/ujson/lib -Ipandas/src/datetime -Ipandas/src/klib -Ipandas/src -IC:\pyth
on27\lib\site-packages\numpy\core\include -IC:\python27\include -IC:\python27\PC /Tcpandas/src/ujson/python/ujson.c /Fobuild\temp.win-amd64-2.7\Release\pandas/src/ujson/python/ujson.obj -D_GNU_SOURCE
ujson.c
C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\Bin\amd64\cl.exe /c /nologo /Ox /MD /W3 /GS- /DNDEBUG -Ipandas/src/ujson/python -Ipandas/src/ujson/lib -Ipandas/src/datetime -Ipandas/src/klib -Ipandas/src -IC:\pyth
on27\lib\site-packages\numpy\core\include -IC:\python27\include -IC:\python27\PC /Tcpandas/src/ujson/python/objToJSON.c /Fobuild\temp.win-amd64-2.7\Release\pandas/src/ujson/python/objToJSON.obj -D_GNU_SOURCE
objToJSON.c
c:\python27\lib\site-packages\numpy\core\include\numpy\npy_deprecated_api.h(8) : Warning Msg: Using deprecated NumPy API, disable it by #defining NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION
pandas/src/ujson/python/objToJSON.c(249) : warning C4267: 'function' : conversion from 'size_t' to 'int', possible loss of data
pandas/src/ujson/python/objToJSON.c(274) : error C2143: syntax error : missing ';' before 'type'
pandas/src/ujson/python/objToJSON.c(275) : error C2275: 'PyDatetimeScalarObject' : illegal use of this type as an expression
        C:\python27\lib\site-packages\numpy\core\include\numpy/arrayscalars.h(123) : see declaration of 'PyDatetimeScalarObject'
pandas/src/ujson/python/objToJSON.c(275) : error C2065: 'obj' : undeclared identifier
pandas/src/ujson/python/objToJSON.c(277) : error C2065: 'obj' : undeclared identifier
pandas/src/ujson/python/objToJSON.c(277) : error C2223: left of '->obval' must point to struct/union
pandas/src/ujson/python/objToJSON.c(277) : error C2065: 'obj' : undeclared identifier
pandas/src/ujson/python/objToJSON.c(277) : error C2223: left of '->obmeta' must point to struct/union
pandas/src/ujson/python/objToJSON.c(277) : error C2065: 'dts' : undeclared identifier
pandas/src/ujson/python/objToJSON.c(277) : warning C4047: 'function' : 'npy_datetime' differs in levels of indirection from 'int *'
pandas/src/ujson/python/objToJSON.c(277) : warning C4024: 'pandas_datetime_to_datetimestruct' : different types for formal and actual parameter 1
pandas/src/ujson/python/objToJSON.c(277) : error C2198: 'pandas_datetime_to_datetimestruct' : too few arguments for call
pandas/src/ujson/python/objToJSON.c(278) : error C2065: 'dts' : undeclared identifier
pandas/src/ujson/python/objToJSON.c(278) : warning C4133: 'function' : incompatible types - from 'int *' to 'pandas_datetimestruct *'
pandas/src/ujson/python/objToJSON.c(356) : warning C4244: '=' : conversion from 'npy_intp' to 'int', possible loss of data
error: command '"C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\Bin\amd64\cl.exe"' failed with exit status 2
PS H:\jreback\pandas>
```
