Add an isAbsolutePath method

Summary: Add a Filesystem::isAbsolutePath method as this functionality is fairly general purpose and reusable.

Test Plan: This logic has just been moved from Filesystem::resolvePath.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: avivey, Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13030