Add install_*() as abstract on base un/installer

See #68
Document the $capabilities property

See #68
Still use $network_active in install file

It is a required argument of `wordpoints_activate()`.

See #68
Use the correct l10n function

See #68
Update the l10n validator config

See #68
Un/installer: add option_prefix property

In favor of several option properties.

See #68
Include WP's upgrade script during install

Makes `dbDeleta()` available for creating databases.

See #68
Un/installer: add a before_install() method

See #68
Tests: account for network activation in simulator

And also in the uninstall tests.

See #68
Add an un_installer component attribute

When a component specifies this argument, it is expected to have an
un/installer class defined in that file, which will be used to
uninstall it.

See #68
Add an un/installer for the points component

See #68
Use an uninstall class for the ranks component

See #68
Automatically install components on activation

…if they have specified an un/installer.

See #68
Update l10n validator config

See #68