Correct fix for #160.

This did in fact turn out to be an oversight in the error handling. Now, any
error in the initial connection path will always trip pg_up to be 0.

Fixes #160

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.

* In docker example both containers should run on the host net 
* remove unecessary portmapping in docs

Adds the following environment variables for overriding defaults:
* `PG_EXPORTER_WEB_LISTEN_ADDRESS`
* `PG_EXPORTER_WEB_TELEMETRY_PATH`
* `PG_EXPORTER_EXTEND_QUERY_PATH`

Closes #150.

Closes #154

Closes #145

Fix Apache license naming.

* fixed version parsing for Windows and EnterpriseDB

* fixed formatting

Revamp the build system to be more inline with other Prometheus exporters.
Notably add Darwin and Windows build targets, and add support for releases
using tar files.

At some point in the past we find/replaced "this" to "cu" in the source code
and it's been like that ever since, including leaking into some of the metric
descriptions. Clean it up.

On advice from @superq this is inline with the convention used in similar
exporters (example being the mysqld exporter). The full metric name is thus
`pg_up`.

Closes #135

This metric is only set to 0 after the database has been connected to and a
Ping() request via the pg driver has been successfully sent.

Closes #135.

Adds a status metric to indicate that the user queries YAML file loaded
successfully. Also adds the file path and hashsum as labels.

Closes #125

When using queries.yaml, extra columns in the output would be detected as
unknown metrics (correctly) but the labels in the current mapping would not be
applied to the output.

This explains the rash of crashes due to inconsistent label cardinality. The
specific creation is when the name given to a column does not match the mapping
you intended, and so the metric shows up as unknown.

Closes #98

In PostgreSQL, views run as the defining user, meaning you don't 
need a view wrapping a function (you can just have a view).  This 
improves readability, maintainability, performance, and clarity.

* Switch to using kingpin for CLI interface as upstream Prometheus has moved
  that way too.

Closes #111.
Closes #91.

Closes #112.