Removes the responsability of sending to pypln from the downloader

As far as I recall, we agreed that it would be better to have another cronjob
to upload files to pypln, instead of making it part of the download process.
Simplifies the code in `send_to_pypln`

Since `pypln.api.Corpus.add_documents` just calls
pypln.api.Corpus.add_document` multiple times
(https://github.com/NAMD/pypln.api/blob/develop/pypln/api.py#L187) this patch
reduces code duplication without including extra complexity.
Fixes bug in send_to_pypln

The `pypln.api.Document` object returned when inserting it in pypln does not
have in `_id` key. I tried to clarify the names of the variables to improve
readability.
Removes `print` statement from `nlp.send_to_pypln`

The idea is to use the `nlp` module as a library, so it shouldn't send data to
stdout.
Removes database access from `send_to_pypln`

Since this module is only supposed to be a wrapper for `pypln.api`, we can keep
database access in the script that will actually get the data from mongo and
send it to pypln.
Don't get corpus for every document

This is one extra request and, since we won't upload documents to different
corpora (the `load` function only takes one corpus name), we don't need to make
this request so many times. We can get the `Corpus` object once and reuse it.
Works around buggy `cursor.count()` behaviour

Adding `with_limit_and_skip=True` to `.count()` should make it return the
number of objects we'll actually iterate through, but this does not happen, so
we just use a counter.
Filters out documents that were already uploaded
Removes batch logic

Since we're sending one document at a time to PyPLN we don't need get the
documents in batches. We can just iterate through the cursor and send the
documents that don't have `pypln_url` set.

I've added command line options to allow users to send small amounts of data if
they want.