Fixed a bug in scheduler driver to properly erase 'savedOffers'.

Review: https://reviews.apache.org/r/22148
Split Authorizer into source and header files.

Review: https://reviews.apache.org/r/22149
Injected Authorizer into Master.

Review: https://reviews.apache.org/r/22150
Changed TaskInfoError and OfferError to Option<Error>.

Review: https://reviews.apache.org/r/22338
Authorized launch tasks.

Review: https://reviews.apache.org/r/22151
Added MockAuthorizer and more authorization tests.

Review: https://reviews.apache.org/r/22190
Authorized resource roles to receive offers from.

Review: https://reviews.apache.org/r/22284