[#4766] wrote tests to indicate behavior is opt-in

- Added `setUseIncludePath()` call to translator adapters
- added flag, Factory::USE_INCLUDE_PATH, as second argument to config
  factory
[#4766] Config makes include_path usage opt-in only

- You must pass the `$useIncludePath` to either `fromFile()` or
  `fromFiles()` for it to be used. Constants were created with boolean
  values for those using IDEs.
[#4766] Make include_path abilities opt-in in Translators

- Created AbstractFileLoader, which provides implementation details for
  when/how to use the include_path to load translation file assets.
- Updated INI, gettext, and PHP loaders to extend AbstractFileLoader.
[#4766] Add docblock detailing how to set include_path flag

- Added to translator loader plugin manager
Remove constants

- Remove constants, per @DASPRiD
Merge branch 'hotfix/4767' of https://github.com/weierophinney/zf2 into weierophinney-hotfix/4767
Merge branch 'weierophinney-hotfix/4767' into develop