Add keyfile and HTTP Basic auth support to repositories

Summary: I still need to go through all the daemon and Diffusion code and change
the bare execx() calls to $repository->execxXXX() to actually make this work,
but we're getting close.

Test Plan: Configured repositories with various HTTP / SVN setups and ran the
test_connection.php script to verify keys were located and added and
username/password information was supplied.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: nh

CC: aran, nh, jungejason

Differential Revision: 902