Control PlacementPolicy at global, Bucket, and Put granularity

[ChristopherHogan]

Closes #162.

• Adds a default_placement_policy option to the configuration file. This becomes the default policy value for all instances of api::Context.
• Save the Context passed to a Bucket. The Context::policy in the Bucket overrides the global default.
• Add overloads of Put that don't take a Context argument. These versions simply use the Context stored in the Bucket.
• Passing a Context to a Put overrides the Bucket's default policy.
• The full Context is preserved on swap blobs when the BO attempts to place them back into the hierarchy. Previously it used a default Context.

[coveralls]

Coverage increased (+0.03%) to 84.958% when pulling e6553f3 on chogan/162 into 9730d23 on master.

[jya-kmu]

Looks good.