dataframe.plot accepts undocumented 'by=' keyword but ignores it.
Maybe this is a request for enhancement, or documentation of a bug - I don't know.  

A tutorial showed that `DataFrame.boxplot()` can take a `by=` keyword to produced plot stratified by the values in one column.   I naively assumed I could apply that to other plots as well.  `DataFrame.plot()` accepts `by=` but seems to silently ignores it.    This seems to be an [undocumented](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.plot.html#pandas.DataFrame.plot) keyword for `DataFrame.plot()`.

Is this intentional?  Why not include the groupby functionality from `boxplot` into `plot` so it can do something reasonable with all the other plot types as well?  If not, then why does `plot` accept the `by=` keyword?

Example code below,

```
import pandas as pd
pd.show_versions()

plt.rcParams.update(pd.tools.plotting.mpl_stylesheet)

# taken from http://pandas.pydata.org/pandas-docs/stable/visualization.html#box-plots
df = pd.DataFrame(rand(10,2), columns=['Col1', 'Col2'] )
df['X'] = pd.Series(['A','A','A','A','A','B','B','B','B','B'])
plt.figure();
print(df)

# boxplot produces two subplots segregated by the values in X
df.boxplot(by='X')

# Other kinds of plots seem to silently ignore 'by='
df.plot(kind='box', by='X')

# Other kinds of plots seem to silently ignore 'by='
df.plot(kind='scatter', x=0, y=1, by='X')

# Other kinds of plots seem to silently ignore 'by='
df.plot(kind='line', by='X')
```

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-32-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.15.2
nose: 1.3.4
Cython: 0.20.1post0
numpy: 1.9.1
scipy: 0.13.3
statsmodels: 0.6.0
IPython: 2.3.1
sphinx: 1.2.3
patsy: 0.3.0
dateutil: 2.4.0
pytz: 2014.10
bottleneck: None
tables: 3.1.1
numexpr: 2.2.2
matplotlib: 1.4.2
openpyxl: 1.8.6
xlrd: 0.9.3
xlwt: 0.7.5
xlsxwriter: None
lxml: 3.3.3
bs4: 4.3.2
html5lib: 0.999
httplib2: 0.9
apiclient: None
rpy2: 2.5.2
sqlalchemy: 0.8.4
pymysql: None
psycopg2: None
       Col1      Col2  X
0  0.561433  0.329668  A
1  0.502967  0.111894  A
2  0.607194  0.565945  A
3  0.006764  0.617442  A
4  0.912123  0.790524  A
5  0.992081  0.958802  B
6  0.791964  0.285251  B
7  0.624917  0.478094  B
8  0.195675  0.382317  B
9  0.053874  0.451648  B
```

![image](https://cloud.githubusercontent.com/assets/295428/5783537/b3232378-9d78-11e4-9cc6-4713944518d0.png)
![image](https://cloud.githubusercontent.com/assets/295428/5783544/c0b007e0-9d78-11e4-9722-d56ee91ebfc2.png)
![image](https://cloud.githubusercontent.com/assets/295428/5783550/ce2b3a5c-9d78-11e4-8bbe-2a01c1639b46.png)
![image](https://cloud.githubusercontent.com/assets/295428/5783553/d52d155a-9d78-11e4-9589-2c39a51f2877.png)
