Implement clearCacheForPage on iOS

[radazzouz]

Details

This PR implements clearCacheForPage for iOS. On Android, we implemented it in #5.

JavaScript Usage:

PSPDFKit.clearCacheForPage(0, function() {
  alert("Cache cleared.");
});

Acceptance Criteria

• When approved, right before merging, rebase with master and increment the package version in package.json and plugin.xml(see example commit: 09d5c6b).
• Create a new release (and tag) with the new package version (see https://github.com/PSPDFKit/PSPDFKit-Cordova/releases).
• Locally, pull the latest master and publish (npm publish) the new release to npm.

[steviki]

LGTM!