Merged PR #21

[getAchievementsAllNames()] Added printing achievement index from JSO…

…N (issue #19);

Fixed initialization of achievements on start (issue #17).

Added getAchievementsQuantity(), getAchievementsAllNames()

[get...() functions] Added checking that argument is in valid range

Fixed serious bug that breaks JSON file on user's side

Little code changes

Added more debug information

Little code changes

Added resetAchievementNotifications()

Added check if userfile buffer is null

Fixed repeating of display achievement notification in another scene

Added getFields functions:

* getFieldName(index)
* getFieldDescription(index)
* getFieldProgress(index)
* getFieldIsSecret(index)
* getFieldIconPath(index)
* getFieldIsHave(index)

[Main]

* For now achievement system works as plugin

[Updated]
* Updated documentation (README)

[Bugfixes]
* Fixed a problem with storing data of achievements that was already got

Changed main node's size to avoid conflict with other UI nodes