Reduce legacy check to one line

Because effected users would have this library loaded in other plugins,
there isn't any point in running `deactivate_plugins` or `wp_die`.

By requiring WordPress 3.2, we also inherantly require PHP5,
so check of PHP_VERSION is not needed.

By checking `$wp_version` outside `gpu_git_plugin_updates_init`, we
can combine all checks into one `if` statement, and don't need
to declare `global $wp_version`.

Because users not running WordPress older than 3.2 *already* have
an update nag displaying, there's no point in displaying an
update notice. Because they're already habitually ignoring upgrades,
it's okay for this plugin to silently not do anything until
WordPress core is upgraded.

Ideas for this commit came about while working through
@afragen's [pull request](https://github.com/afragen/git-plugin-updates/commit/d0315f6c95711d2ccece5068b94536f152b17242) for issue #12.
Move if statement above constant definitions, because @GaryJones [is a genius](https://github.com/brainstormmedia/git-plugin-updates/issues/12#issuecomment-27470809).
Reorder safety checks in order of liklihood. See https://github.com/brainstormmedia/git-plugin-updates/commit/b5abd85a7a3df7aff9d7f603ff8caa9aafc2da2f#commitcomment-4475221
Remove GPU_PLUGIN_DIR constant
Replace text-domains constant with string.
Remove GPU_Controler::__get and . Not being used for anything. Resolves #7.
Merge branch 'develop' into f/12-wp-version-check
Update paths in readme. Reword usage instructions.
Merge pull request #16 from brainstormmedia/f/12-wp-version-check

Reduce legacy check to one line. Use strings for text-domains. Remove GPU_PLUGIN_DIR constant.
Merge branch 'develop' of github.com:brainstormmedia/git-plugin-updates into develop
Cleanup readme code snippets. Add @GaryJones to credits.
no need to use variable, plus it seems to be empty.
use actual domain as this is always going to be 'bitbucket.org'
at request of @pdclark
Merge pull request #27 from afragen/bugfix/github-uri

no need to use variable, plus it seems to be empty.
Simplify init; Force plugin to override library

Simplify loader:
* Move constants to class constants.
* Remove init function.

Use as library:
Update readme with code for users adding as library.
Merge branch 'develop' of github.com:brainstormmedia/git-plugin-updates into develop
Ignore PluginURI header for updates by default.
Remove log method. Not being used.
Version bump. Changelog. Add License URI and Git URI.
Merge branch 'r/2.0.1'

Conflicts:
	git-plugin-updates.php
	readme.md