Improve the "ext enable" command
Improve the "ext enable" command

Currently, the `ext enable` command cannot enable an extension if it was not installed by PHPBrew (only the *.so file exist). This would be useful in the following cases:

1. An extension is built as shared as part of `phpbrew install`. Some extensions like [`opcache`](http://php.net/manual/en/opcache.installation.php#opcache.installation.bundled) and [`gd`](https://github.com/morozov/phpbrew/blob/dc903cccc2dfe45c011b717bd98d1ebab1624e73/src/PhpBrew/VariantBuilder.php#L393) cannot be compiled statically with PHP and thus have to be enabled explicitly after installation.
2. A pre-built extension (e.g. blackfire or newrelic) is downloaded and needs to be enabled by means of PHPBrew.

The first case could be improved even better to the extent that if an extension is built as shared implicitly (opcache) or explicitly (`--with-gd=shared`), it's enabled automatically (same as if it was compiled statically).

Additional changes:

1. `ExtensionManager::createExtensionConfig()` now returns a boolean result.
2. .ini files for a zend extension are now created with absolute path for PHP versions older than 5.5 where it's necessary (see https://github.com/php/php-src/commit/0def1ca59a). Having a relative path is more convenient when *.ini files are manually copied between PHP builds.
3. The generated .ini file now contains "\n" at the end of line.

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