DOC: make api docs reflect api usage (#4539)
Closes #4539.

This removes all use of `.. currentmodule:: pandas.some.submodule` (in favor of `.. currentmodule:: pandas`) in api.rst. Rationale is that docs should reflect the user api usage.

Only exception are `pandas.io.sql.read_frame/write_frame` and `pandas.io.stata.StataReader` methods, because they are not available in the top-level pandas namespace.

All old links will remain valid because they are still generated because I added all those in a comment at the bottom in api.rst (only not for Index, because this was only added very recently in api.rst).  
The **downside** with this approach is that you now get warnings when building the docs (`document isn't included in any toctree` for these functions). I am not sure how I can prevent this while keeping the old links but not including it visually in api.rst.

OK, solved the downside by adding a hidden toctree. So now all old links remain valid, and no warnings are generated. The only downside is that there is a big chunk of lines at the bottom in api.rst that is maybe somewhat noisy, just to keep the links.
 If you can add comments, can you leave a note saying that its' there to maintain links?
 I did, quoting from the commit 

```
HACK - see github issue #4539. To ensure old links remain valid, include
here the autosummaries with previous currentmodules as a comment and add 
them to a hidden toctree (to avoid warnings):
```

If it is not clear enough, please suggest something.

Travis had a failing (related to sql: `test_basic (pandas.io.tests.test_sql.TestMySQL)`. Strange, because it is not related to the docs, and before I updated the commit (a forced push), the commit was green. Is this a problem?
 yep unrelated
