Extracted generic methods from Docker::Image.
Add Docker::Image#insert_local.

Since Docker::Image#insert only accepts a URL, another method was added
to add local files.
Bump the max abc complexity.
Updated Docker::Image#insert_local to be more like Docker::Image#insert.
Patch version bump (1.5.2)
Merge pull request #35 from swipely/add-insert-file

Add insert file