Add Docker::Container#copy.

Given a file or directory, will yield each chunk of it.
Version bump 1.5.1 (patch version bump).
Added documentation for Docker::Container#copy.
Made the variable name in Container#copy more clear.

Since Container#copy may output either a file or directory, it was
renamed from file to path.
Added documentation for Docker::Container#delete.
Merge pull request #34 from swipely/add-container-copy

Add container copy