PERF: msgpack encoding changes to use to/from string for speed boosts

API: disable sparse structure encodings and unicode indexes

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
packers_read_pack                            |   3.5113 |  17.8316 |   0.1969 |
packers_read_pickle                          |   0.6769 |   0.6770 |   0.9999 |
packers_write_pack                           |   1.8814 |   3.5230 |   0.5340 |
packers_write_pickle                         |   1.5387 |   1.4664 |   1.0493 |

packers_write_hdf_store                      |  12.1900 |  12.2033 |   0.9989 |
packers_read_csv                             |  52.3347 |  52.2310 |   1.0020 |
packers_write_csv                            | 536.2056 | 526.5187 |   1.0184 |
packers_write_hdf_table                      |  33.3436 |  32.4137 |   1.0287 |
packers_read_hdf_store                       |   8.3120 |   8.0493 |   1.0326 |
packers_read_hdf_table                       |  13.9607 |  12.9707 |   1.0763 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [381e86b] : PERF: msgpack encoding changnes to use to/from string for speed boosts
API: disable sparse structure encodings and unicode indexes
Base   [46008ec] : DOC: add cookbook entry
```

```
In [1]: from pandas.io.packers import pack

In [2]: import cPickle as pkl

In [3]: df = pd.DataFrame(np.random.rand(1000, 100))

In [6]: %timeit buf = pack(df)
1000 loops, best of 3: 492 ﾵs per loop

In [7]: %timeit buf = pkl.dumps(df,pkl.HIGHEST_PROTOCOL)
1000 loops, best of 3: 681 ﾵs per loop

In [8]: df = pd.DataFrame(np.random.rand(100000, 100))

In [11]:  %timeit buf = pack(df)
1 loops, best of 3: 184 ms per loop

In [12]: %timeit buf = pkl.dumps(df,pkl.HIGHEST_PROTOCOL)
10 loops, best of 3: 111 ms per loop
```

now pretty competitive with pickle

note on bigger frames, writing an in-memory hdf file is quite fast
```
In [3]: def f(x):
   ...:     store = pd.HDFStore('test.h5',mode='w',driver='H5FD_CORE',driver_core_backing_store=0)
   ...:     store['df'] = x
   ...:     store.close()
   ...:     

In [11]: df = pd.DataFrame(np.random.rand(100000, 100))

In [13]: %timeit -n 5 buf = pack(df)
5 loops, best of 3: 202 ms per loop

In [14]: %timeit -n 5 buf = pkl.dumps(df,pkl.HIGHEST_PROTOCOL)
5 loops, best of 3: 115 ms per loop

In [15]: %timeit -n 5 f(df)
5 loops, best of 3: 53.9 ms per loop
```