Support writing unicode characters in df.to_stata()
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.DataFrame({'a': ['丆']})
df.to_stata('test.dta')
# UnicodeEncodeError: 'latin-1' codec can't encode character '\u4e06' in position 0: ordinal not in range(256)
```
I picked an arbitrary CJK character to test this with.

#### Problem description

It would be possible to write Unicode strings to a Stata file by implementing a writer according to [version 118](https://www.stata.com/help.cgi?dta) of the `dta` format.

~~I'd be interested in trying to submit a PR for this.~~ (Edit: I don't use Stata anymore)

#### Expected Output

Stata file written to disk.

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

<details>

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 2.6.32-696.18.7.el6.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 3.5.1
pip: 10.0.1
setuptools: 39.1.0
Cython: 0.28.2
numpy: 1.14.3
scipy: 1.1.0
pyarrow: 0.10.0
xarray: None
IPython: 7.0.1
sphinx: 1.7.4
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.3
numexpr: 2.6.5
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.3
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.4
lxml: 4.2.1
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: 0.1.6
pandas_gbq: None
pandas_datareader: None
```

</details>
