WIP: Added the method export_to_csv and its relevant cli

Types of changes

What types of changes does your code introduce to ether_sql? Put an x in the boxes that apply

[ ] Bugfix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Explain your changes

This is a response to the issue: https://github.com/analyseether/ether_sql/issues/15

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

[x] I have read the CONTRIBUTING doc
[ ] Lint and unit tests pass locally with my changes
[x] I have added tests that prove my fix is effective or that my feature works
[ ] I have added necessary documentation (if appropriate)

Any relevant logs, error output, etc?

Sometimes the tests pass but several times they get stuck. I think the issue is due to not properly closing the session of sqlalchemy. https://stackoverflow.com/questions/26350911/what-to-do-when-a-py-test-hangs-silently?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa

(ether_sql) ankit@ankit-X541UAK:~/projects/ether_sql$ python -m pytest -vv tests/infura/test_sql.py 
============================================================= test session starts =============================================================
platform linux -- Python 3.5.5, pytest-3.0.0, py-1.5.3, pluggy-0.3.1 -- /home/ankit/anaconda3/envs/ether_sql/bin/python
cachedir: .cache
rootdir: /home/ankit/projects/ether_sql, inifile: 
plugins: cov-2.2.1
collected 4 items 

tests/infura/test_sql.py::TestEmptyDB::test_create_db PASSED
tests/infura/test_sql.py::TestEmptyDB::test_remove_db PASSED
tests/infura/test_sql.py::TestEmptyDB::test_block_number PASSED
tests/infura/test_sql.py::TestEmptyDB::test_export_to_csv

The tests are stuck on the last step since forever.

Any other comments?

This pull request is still a work in progress

Where has this been tested?

Operating System: Ubuntu 16.04 Node: Parity

analyseether / ether_sql

WIP: Added the method export_to_csv and its relevant cli #20