ENH: Adding '.' as an na_value for FRED.
The St. Louis Fed's [FRED](http://research.stlouisfed.org/fred2/) uses '.' as an NaN marker.  I don't think the user has any way to specify the na_value via DataReader, so this just hard codes '.' as an na_value in the fred data-fetching function.

Before:

``` python
In [4]: from pandas.io.data import DataReader

In [5]: df = DataReader('DFII5', data_source="fred")

In [6]: df.head()
Out[6]:
           DFII5
DATE
2010-01-01     .
2010-01-04  0.52
2010-01-05  0.44
2010-01-06  0.44
2010-01-07  0.43
```

After

``` python
In [2]: from pandas.io.data import DataReader

In [3]: df = DataReader('DFII5', data_source="fred")

In [4]: df.head()
Out[4]:
            DFII5
DATE
2010-01-01    NaN
2010-01-04   0.52
2010-01-05   0.44
2010-01-06   0.44
2010-01-07   0.43

In [5]: pd.__version__
Out[5]: '0.11.0rc1'
```

Hopefully I've done everything correctly with the pull request.  Let me know if anything needs fixing.

I see you haven't got travis going, is this covered by the test suite? if not, please add a test to
monitor this in the future, after that I have no problem merging.

thanks.
 Thanks, I'll get a test going added and I'll try to get travis set up also.  I've never used it before.

One question: I could be missing it, but I'm not sure the fred code is covered by any tests.  I greped through the pandas/io/tests/ directory and I didn't see any references to fred.  Should I start a new file similar to the ones for yahoo and wb?  I can add some more tests while I'm in there too.
 Yeah, that's not a part of the code I'm familiar with, it's possible. In which case
you would be doing pandas an even greater service by throwing together some tests.
 [CONTRIBUTING.MD](https://github.com/pydata/pandas/blob/master/CONTRIBUTING.md) has a link to the travis instructions, and other stuff if you
have the patience to read through it.

remember to tag your tests @network(+@slow?)
 related #3413 
 OK I think Travis is testing my branch now.  Assuming that passes, do you want me to try to stuff both my changes (handling the NaNs and adding the tests) into a single PR?  Or should I do them separately?
 Same PR makes sense, feel free to add as many commits as you like to the PR branch and
push, just make it clear whether it's ready for merging or still WIP.

If your git-fu is good, you can rewrite and force push and all that in PR branches, that's
acceptable, if not, just add commits and push and I or another dev will squash things
down for you before merging, that's not a problem.

Thanks.
 I added a check for if the name you gave is a valid FRED series name, and a test that goes along with it.  I'm worried that it will break if the St. Louis Fed decides to change their response to an invalid name.  But the way I coded it, it should only occur if we already failed to return a dataframe.  It's basically just a more informative error message.

I'd like to take a crack at #3413, but I might not have time until later this week.  It should be something as simple as changing the fred branch of the main DataReader function to

``` python
names = list(name)  # if string is passed for a single series
return pd.concat([get_data_fred(name, start, end) for name in names])
```

I may let someone else handle the history rewriting.  Last time I tried that over on statsmodels, I think I gave Joseph a headache.  Travis is testing now.  I'll add the final bit and mark it as ready to merge when it's done.
 OK This should be ready to go if you think it looks acceptable.

Thanks for the help y-p.
 One last thing, could you add a note to RELEASE.rst, to let users know you "fixed it ™"?
 Will do.  I've just noticed that I'm on `0.11.0rc1` for my master, so I've got an out of date RELEASE.rst; CONTRIBUTING.md says "Don't merge upstream into a branch you're going to submit as a PR".  Am I ok doing

```
git checkout master
git rebase upstream/master
git checkout fred_na  # my branch
git rebase master
```

And then adding the changes to RELEASE.rst and pushing again? Sorry.
 Exactly right. some people have a hard time grokking rebase. but that's the way
you should it. You can also just leave it in, and let us handle the conflicts if you don't
feel confident.
 Well I think I messed something up. After fetching and rebaseing my master onto upstream/master, when I did `git checkout fred_na` and `get rebase master` things seemed to work correctly, but now I'm told that 

```
# Your branch and 'origin/fred_na' have diverged,
# and have 121 and 3 different commits each, respectively.
#   (use "git pull" to merge the remote branch into yours)
```

So I did `git pull` but now it looks like all of those other commits are going to be part of this PR when I push, which we don't want right?
 first, don't panic.

Most important git command in existence:

```
git reflog
```

Coupled with

```
git reset --hard commithash
```
 that error message is actually fine. origin has the previous state based on 0.11.0rc1,
your local branch has been rebased. so they have diverged.
your local is correct, and the remote needs to be clobbered.

write down the commit hash that's "safe" to go back to: a80acc4
then clobber the GH branch with your updated local branch (only allowed
on your own PR branches, not generally) with:

```
git push origin *branchname* --force
```
 I panicked :)

Seems to look ok now I think, once Travis is done?
 Everything passes locally on my box. in it goes...

The log shows this is your first PR to pandas. good job! :+1: 
