Adds unit tests for db_task_history, bug fixes

Uses sqlite3 in memory mode for db.
If sqlalchemy can't be imported, skips the test rather than failing.
Server logs with timestamp archived in daily files
Merge pull request #156 from spotify/freider/server_log_spooling

Server logs with timestamp archived in daily files
Merge pull request #151 from JoeEnnever/db_fix

Adds unit tests for db_task_history, bug fixes
added File.copy

Use case: when a file, e.g. from an ExternalTask lives
on a read-only filesystem, you cannot `move` it.

One unittest included.
Merge pull request #158 from miku/feature/filecopy

added File.copy
Add Scalding support

Change-Id: I5c3ef04d7cc2db8cfb5944875bbeecb95b5d86e0
Merge pull request #110 from spotify/neville/scalding

Add Scalding support