BUG: `read_stata` always uses 'utf8'
#### Code Sample, a copy-pastable example if possible

```python
import pandas
data = pandas.read_stata(file_with_latin1_encoding, chunksize=1048576)
for chunk in data:
    pass # do something with chunk (never reached)
```
This raises `UnicodeDecodeError: 'utf8' codec can't decode byte 0x?? in position ?: invalid start byte`.
OK. So the file isn't a **utf8** one. Even though the StataReader doesn't specify any Unicode support; I then try and open it with a **latin-1** encoding:
```python
import pandas
data = pandas.read_stata(file_with_latin1_encoding, chunksize=1048576, encoding='latin-1')
for chunk in data:
    pass # do something with chunk (never reached)
```
This raises the same exception at exactly the same place (still **utf-8**).

#### Problem description

This is a problem because it appears that `read_stata` doesn't honour the `encoding` argument.
I think this line introduced a bug. The `StataReader` doesn't manage any other type of data than **ascii** or **latin-1**.

Changing the line **1338** of the `pandas.io.stata` module:
```python
        return s.decode('utf-8')
```
to:
```python
        return s.decode('latin-1')
```
Seemed to make everything work and I could read the data from the given file.
Even better, changing it to the following:
```python
        return s.decode(self._encoding or self._default_encoding)
```
also seems to have made it work.

I believe though, that if you want to make this work with **Unicode** too you'd have to add the following encodings to `VALID_ENCODINGS`: **utf-8**, **utf8**, **iso10646**.

#### Expected Output
The file should be correctly read and parsed

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.10.0-37-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: ro_RO.UTF-8
LANG: ro_RO.UTF-8
LOCALE: None.None

pandas: 0.24.0.dev0+41.gb2eec25
pytest: 3.2.3
pip: 9.0.3
setuptools: 36.6.0
Cython: 0.28.2
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 5.1.0
sphinx: 1.6.3
patsy: None
dateutil: 2.7.3
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: 2.4.9
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: None
lxml: 3.8.0
bs4: None
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
