Allows the creation of multiple pipelines at once

This means the manager is only created once, so we'll connect much fewer times
when more than one document is uplodaded.

Fixes #48 and helps with #43 (there seems to be some other bottleneck for that
one; in my development environment it takes a long time to process lots of
files before they hit the send pipelines step, but maybe in the server this
will be less noticeable).
Removes worng return statement

We're not using the pipeline id for anything, and who would ever commit a for
loop with a return in it?
Return created pipeline id's
Groups all 'tag not found' messages in one e-mail

Sending more than one e-mail per document was not only unnecessary but also
slowed down the visualization generation a lot.
Fixes KeyError in wordcloud visualization for Unknown languages

When trying to filter stopwords the visualization would not find an empty key
in the dictionary, raising a KeyError
Merge pull request #57 from flavioamieiro/bugfix/unknown-language-in-wordcloud-visualization

Fixes KeyError in wordcloud visualization for Unknown languages
Merge pull request #56 from flavioamieiro/bugfix/missing-tag-emails

Groups all 'tag not found' messages in one e-mail
Merge pull request #53 from flavioamieiro/bugfix/improve-send-pipelines

Allows the creation of multiple pipelines at once