Add context manager for Mock.open().

The HdfsTarget's support this, so it makes sense to add to mocks
for compatibility.

Also snuck in a change to gitignore for some MacOS files.
Merge pull request #74 from jcrobak/mock-sb-context-man

Add context manager for Mock.open().
Fixes errors in handling already existing files on .
Also adds unit test to test this functionality
Merge pull request #75 from spotify/freider/mkdir_exists_handling

Fixes handling already existing files in hdfs.mkdir
better logging output for hive targets
Merge pull request #78 from rathboma/master

Minor change -- debug output for hive targets
Merge pull request #76 from JoeEnnever/hive_fix

Fixes MetastoreClient.table_exists for partitions
Fix relative paths in webhdfs.

This fixes relative paths in webhdfs by prepending
the users home dir whenever the path is relative.
Merge pull request #79 from jcrobak/webhdfs-fixes

Fix relative paths in webhdfs.
New luigi task status and graph visualiser (squashed for cleaner history, including commits by lallea, Anders Nyman and Uldis Barbans)
New hand-written layout engine more suited to directed acyclic graphs (like dependency graphs). Backend unit tests now use scheduler directly instead of RPC.
Killed old visualiser and fixed 2.7-dependent tests
Visualiser now fetched error traces from luigi. Failed nodes in graph are clickable for a traceback. Ships with mock data for testing UI in isolation
Added root path handler to redirect / to /static/visualiser/index.html
Merge pull request #80 from DavW/new-visualiser

New visualiser to display failing tasks and dependency graphs
Added recursive scan to setup.py to get round limitations is python's packaging
Changed to using relative paths for API calls. Useful if luigi entry-point root is changed to something other than /
Merge pull request #81 from spotify/freider/visualiser_relative_paths

Changed to using relative paths for API calls
Make MockFile a FileSystemTarget, add FileSystem.

Barebones "FileSystem" implementation for MockFile. Some semantics
don't 100% match hadoop fs, but this is a good start for testing.
Merge pull request #82 from jcrobak/mockfile-fs

Make MockFile a FileSystemTarget, add FileSystem.