Feat/sql types alt

[tools4origins]

Same as #97 but with a rewritten history, it's on top of #101 which is on top of #100, reviewing them in order is probably much easier:

This PR:

• Introduce Spark SQL types
• Introduce SQL-related exceptions
• Adds a python_to_spark_type function which maps a python type to its spark counterpart

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[tools4origins]

Added 2 commits:

• 1 that seems to fix a transient issue in multithreading tests, which seems to have been caused by too many thread/executor pools creations with regard to Travis CI machine resources (I was able to reproduce it on my laptop by increasing the pool size).
• 1 that explicit that a new pylint warning is not relevant

Travis is green :) https://travis-ci.org/github/svenkreiss/pysparkling/builds/681921094

Do you think we could set pylint version in setup.py so that we don't have those new warnings appearing in unrelated code modifications?

PS: Added a rebase

[svenkreiss]

Great. Also thanks for fixing the multiprocessing pool tests!

About fixing pylint version: I am open to that. I would be against getting too far behind so if the pylint version is fixed, it should still be updated from time to time, but that could be done in a PR that does just that.