Clarify sourcemap example with postcss.parse

Without comment line confusingly looks like obligatory action.
Merge pull request #90 from princed/patch-1

Clarify sourcemap example with postcss.parse
Documentation cleanup.

Mostly grammar, punctuation and spelling changes.
Merge pull request #91 from markfinger/master

Documentation cleanup
Add tokenizer
Add errors tests to tokenizer
Better tests for tokenizer
Add content value to all tokens
Do not split space tokens
Add end position for word tokens