Refactor BoxFoldersResourceManager to follow the style in BoxFilesResourceManager

When BoxFilesResourceManager was introduced, it bought with it some additional abstractions
for creating API operations and makes the process of creating a new endpoint more DRY.
Bring these changes to BoxFoldersResourceManager to make the SDK code more consistent.

This commit also factors out magic strings to constants for both internal and external
use.
Rename subresource constants and fix typo in method name in folders manager
Rename subresource constants and reformat files manager

Add linebreaks to long method invocations to make the code easier to
read.
Merge pull request #21 from box/refactor/folders-resource-manager

Refactor BoxFoldersResourceManager to follow the style in BoxFilesResourceManager