Document requirements for SNI support on Python2

A section for Request's advanced usage guide on what Server Name
Indication is, its purpose, and how to enable it on Python2.
Move SNI documentation to FAQ

Relocate documentation on Server-Name-Indication from the advanced
section to the frequently asked questions. This is minus details on
enabling SNI on Python2, which is instead captured by linking to Stack
Overflow.
State early in SNI discussion Python2 limitation

Reference at the earliest opportunity that Server-Name-Indication is a
limitation of Python 2.6 and 2.7. Avoid describing it as a Requests
issue.
Merge pull request #1897 from aliles/master

Document requirements for SNI support on Python2