English grammar and typo fixes - 01.0.md to 04.6.md

Hey there, just want to say nice book! It's a great intro to building Go webapps.

I went ahead and corrected a few typos/grammatical errors in the English version as I was reading it.
I tried not to change any of the original content too much, except when the meaning was unclear. 

Feel free to merge any changes. I'll be editing the rest of the book as I go through it.