Tweak layout in README.md installation markup.

Some code blocks weren't indented, so they ended up messing up the numbering. Indenting them fixed it.
update version to 2.6.2 to fix broken tarball on npm #506
Merge pull request #832 from craigcitro/patch-1

tweak layout in readme installation markup
Fix small misspelling: Mardown -> Markdown
Merge pull request #883 from seanfisk/fix-small-misspelling

fix small misspelling: Mardown -> Markdown