Merge pull request #1 from codeaditya/master

Conformity with PEP-0008 Guidelines
Added support for downloading delivery data.
Fixed 403's due to NSE's user agent detection.

Added Requests' sessions which use keep-alive, should reduce download times.
Plus some minor clean up.
Revert "Added support for downloading delivery data."

This reverts commit 8258d0713d9c45fad3b362487c272643a561a06d.
The delivery data columns were messing up the MetaStock data downloader.