build/tools: Install pipbot binary

Issue #178.
tools/pipbot: Remove 'wtf' command

Issue #178.
tools/pipbot: Add 'build summary' command

This supersedes the wtf command.

Issue #178.
configure: Set a default PREFIX

We want PREFIX to default to the user's ~/.local directory, since we
don't want to use /usr/local as that'll mean we need sudo permissions to
install.

Issue #178.
tools/pipbot: Rename is_number() to is_int()

Issue #178.
tools/pipbot: Add a 'show' command

Issue #178.
tools/pipbot: Make build_target() toplevel

Issue #178.
Merge branch 'feature/178'