Copy updates: Corrected some typos and misspellings, fixed some grammar mistakes, and added some hyphens (hopefully not too many)

Here's a timeline, just in case anyone is curious:
1. Checked the WhisperSystems GitHub landing page to see if TextSecure and TextSecure-iOS have had any updates (to say that I'm excited about v2 and the iOS release is a massive understatement)
2. Noticed a new whispersystems.org repository
3. Saw several blog posts that I hadn't read before
4. Visited the Whisper Systems home page, started reading, and came across a typo
5. Forked the repository and made the fix
6. Figured I may as well read through the entire site

Anyway, I hope this helps!