Use NS_ENUM (+ Doc Fixes)

Hi Tony,

This PR does the following:

* Replaces `enum` with the [`NS_ENUM` macro](http://nshipster.com/ns_enum-ns_options/) for "type-checking and `switch` statement completeness" improvements.
* Fixes up the README quite a bit in terms of spelling, grammar, and in two cases Markdown syntax.
* Fixes up the capitalization and grammar in the comments of `Reachability.m`. They're picky fixes, but overall they make everything more readable (and we're already doing this PR anyway).