Add support  of 'decimal' option to Series.to_csv and Dataframe.to_csv
closes #781

The 'decimal' option exists for read_csv method but not yet in 'to_csv' methods.
The lack of this option is particulary painful when we _have to_ work with Excel with European regional settings. 

This modification add this option to both Series.to_csv and Dataframe.to_csv.

along those lines, would be nice to add a vbench for this as well (and then you'll know!)
see [here](https://github.com/pydata/pandas/wiki/Performance-Testing)
 @jreback Can this be revisited? 

Is there something that needs to be changed by @bertrandhaut (apart from adding a vbench)
 I think this needs a test?
 As suggested, I've added a test for this new option.
 Joris' comments taken into account
 @jreback Is this good to go? (not really familiar with this)

@bertrandhaut Before merging, we ask to squash the commits into one. Can you do that (https://github.com/pydata/pandas/wiki/Using-Git#fetch-and-then-rebase-interactively-to-squash-reword-fix-otherwise-change-some-commits, if you have questions, just ask!)
 @bertrandhaut can you do a vbench run on the csv's to make sure that perf is unchanged, see here:https://github.com/pydata/pandas/wiki/Performance-Testing
 For the vbench I tried but without success. For information I've only python3 on my computer and it seems that vbench is still not ported to python3.
 About the vbench, I think that is correct that is not yet ported to python 3.
 @bertrandhaut can you rebase and squash to a single commit. looks ok otherwise to me.
 @bertrandhaut You were indeed working on master. In the future, better to always first make a branch if you are going to work on a certain feature/fix. But for this PR, it is OK.

Given you are working on master, this should work to rebase/squash:

```
git fetch upstream
git rebase -i upstream/master
```
 merged via https://github.com/pydata/pandas/commit/671c4b3c52a02ee3736752b2645d08762c0fedaf

thanks!
