reorganize scripts/ directory

Summary:
This is a cosmetic change and moves some more "targeted" scripts to specific
directories. It also pairs Windows-specific scripts with their Posix
counterparts.

Reviewers: jacko

Reviewed By: jacko

Differential Revision: https://phabricator.buildinspace.com/D138
stop claiming that git submodules break bisect

Summary:
If a peru project doesn't put `peru sync` inside its build tools (or if
it has no other build tool), the user still needs to call it in every
step of a bisect, or after a checkout. In this way, it's no better than
`git submodule update`. There maybe be a point about it being more
appropriate for Make to call peru than for it to call git, but I'm not
really sure. Let's just take this part out of the readme.

See: https://www.reddit.com/r/vim/comments/2o6av8/peru_pathogen_a_new_alternative_to_plugin/cmlc4l0

Closes https://github.com/buildinspace/peru/issues/64.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D137
fix the travis build

Summary: We forgot to point .travis.yml to the new test script name.

Test Plan: Pushed a branch to my fork.

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D140
create docs/architecture.md

Summary:
A first shot at documenting how peru works internally, particularly the
plugin interface.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D139