Rewrite README documentation

Reorganize and simplify the contents of the README, pending more
complete CLI and online documentation.

Provide a logical order to the README documentation, greater clarity
around Bower's key commands, and consolidate all the disparate
information about the manifest and configuration file.
readme - move `-g` flag to after the command. Fixes #346
Write json in an async way and caught errors, fixes #352.
Typo.
Merge branch 'master' into package-json-name-is-authoritative