Ignore comment lines in read_csv parsing
This is the first part of #7470.
closes #2685 

@amras1 pls squash, otherwise looks ok

@cpcloud @jorisvandenbossche ?
 not familiar with the c parser code, so can't comment on that, but added some doc comments.

@amras1 You updated the io.rst explanation of the parameters, but I think you forgot to also update the parameter explanations in the docstring (it was in your previous PR I think, so probably just have to copy it)
 @jorisvandenbossche - Oops, I'll update that now. Let me know if there are any other doc problems.
 looking good what the docs is concerned.

Maybe you could also add a test for the combination of using comments with skiprows and header? (or are there already?)
 Ok, updated with a skiprows/header test
 @jorisvandenbossche ok?
 @amras1 thanks!
 @jreback - You're welcome!
 #7470 can do for 0.15.0 as its an API change
 give a check on the docs when they are built (shortly): http://pandas-docs.github.io/pandas-docs-travis/io.html#comments and verify how it looks..thxs
 Sure thing
 Weird...there's one section in which ipython outputs this:

```
In [15]: data = 'a,b,c\n# commented line\n1,2,3\n#another comment\n4,5,6'

In [16]: print(data)
a,b,c
1,2,3
4,5,6

# commented line
#another comment
```

The rest of the output looks fine.
 Indeed strange. I suppose this is an issue with the processing in the ipython directive. If you want to look into it, the code is in https://github.com/pydata/pandas/tree/master/doc/sphinxext/ipython_sphinxext (but which is a copy from ipython itself), or you can open an issue at ipython.
