Refactor repository layout and convert build system to Mage.

This commit implements a massive refactor of the repository, and
moves the build system over to use Mage (magefile.org) which should
allow seamless building across multiple platforms.

.gitignore

@@ -1,15 +1,15 @@
-.build
-postgres_exporter
-postgres_exporter_integration_test
+/.build
+/postgres_exporter
+/postgres_exporter_integration_test
 *.tar.gz
 *.test
 *-stamp
-.idea
+/.idea
 *.iml
-cover.out
-cover.*.out
-.coverage
-bin
-release
-*.prom
-.metrics.*.*.prom
+/cover.out
+/cover.*.out
+/.coverage
+/bin
+/release
+/*.prom
+/.metrics.*.*.prom