boxplot fontsize argument only works on x axis (and not on y axis)
#### Code Sample

```python
import pandas as pd
df = pd.DataFrame({"a": [0, 0, 0, 1, 1, 1], "b": [1, 2, 3, 4, 5, 6]})
df.boxplot("b", by="a", fontsize=20)
```
#### Problem description
The fontsize is only applied to the x axis and not to the y axis tick labels (see figure).
![boxplot](https://cloud.githubusercontent.com/assets/4377312/21848879/4522967a-d803-11e6-908a-becdc1739e40.png)

#### Expected Output
IMO this argument should change the font size of both axes tick labels. Side note: I've tried to use the new plot API, but I couldn't get the output to match the one shown above using `df.plot.box` (I couldn't figure out how to set the variable to plot in the first place - probably this only works on Series and not on DataFrames).

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

<details>

    INSTALLED VERSIONS
    ------------------
    commit: None
    python: 3.5.2.final.0
    python-bits: 64
    OS: Darwin
    OS-release: 15.6.0
    machine: x86_64
    processor: i386
    byteorder: little
    LC_ALL: en_US.UTF-8
    LANG: en_US.UTF-8

    pandas: 0.18.1
    nose: 1.3.7
    pip: 8.1.2
    setuptools: 27.2.0
    Cython: 0.24.1
    numpy: 1.11.1
    scipy: 0.18.1
    statsmodels: 0.6.1
    xarray: None
    IPython: 5.1.0
    sphinx: 1.4.6
    patsy: 0.4.1
    dateutil: 2.5.3
    pytz: 2016.6.1
    blosc: None
    bottleneck: 1.1.0
    tables: 3.2.3.1
    numexpr: 2.6.1
    matplotlib: 1.5.3
    openpyxl: 2.3.2
    xlrd: 1.0.0
    xlwt: 1.1.2
    xlsxwriter: 0.9.3
    lxml: 3.6.4
    bs4: 4.5.1
    html5lib: None
    httplib2: None
    apiclient: None
    sqlalchemy: 1.0.13
    pymysql: None
    psycopg2: None
    jinja2: 2.8
    boto: 2.42.0
    pandas_datareader: None
</details>
