The configure command writes out cred vars to shared credentials file

Fixes #847.  The change is implemented as specified in the issue:

* Anytime you set credential variables (access_key/secret_key/session_token)
  using the `configure` command or the `configure set` command, the
  values are always written to `~/.aws/credentials`.
* Getting access_key/secret_key/session_token will look in the
  shared credentials file first before looking in the CLI config file.

This does *not* do anything with automatically migrating over to the
shared credentials file, that is, if you have credentials in the CLI
config file and you run `aws configure` and hit enter 4 times, we
will not write out values to the shared credentials file because
there are no new values to write out.

cc @danielgtaylor @kyleknap 
Grammar: `are specified`?