Allow adding help to existing modules

Currently helpinject produces standalone modules, which have valid use cases, but in many cases it would be useful to add help to existing modules. Related issue: #428624: Drush support, including adding help to an existing module → .

Proposed approach:

• Create new module, helpinjector, which implements hook_help() and hook_form_alter(). Implementations simply call hook_helpinjector_help() and drupal_alter('helpinjector_form'). Also includes versions of the helper functions and CSS currently put into each helpinjector-created module.
• Declare constants, HELPINJECT_EXPORT_DEPENDENT and HELPINJECT_EXPORT_STANDALONE
• In export, if mode is HELPINJECT_EXPORT_DEPENDENT:
  • skip some of the functions (those moved to helpinjector)
  • adjust function calls and classes to use helpinjector versions.
  • put hook_helpinjector_help() and hook_helpinjector_form_alter() in modulename.hellpinjector.inc file
  • .module file calls module_load_include() for .inc file