Major re-write of sections no longer up-to-date

Updated much of content to be consistent with 2.6 MongoDB, added
placeholder for Aggregation section.
Aggregation changes and more fixes

Removed most of MapReduce, added section on Aggregation Framework,
additional fixes.
Syntax updates

added note about 2.6 update
Added 2.6 update text
Spelling corrections
Incorporating first set of feedback on verbiage

More consistent phrasing, spelling/word corrections, amending some
changes.
Incorporating second set of feedback comments
updates based on most recent feedback
Merge pull request #38 from asya999/master

Updates for 2.6