Some details at README

Some typos and other improvements

- Don't repeat usage at Features section
- Add missing TravisCI URL
- Code typo at line 96
- At Positionals, change precision to show how numbers are rounded
- Little grammar/formatting fix at line 168
- Link to Mocha
- Codeblocks and tags which aren't separated by one blank line could cause problems (lines 1, 172 and 178)