Added getAttributePool, getRevisionOfHead and getRevisionChangeset methods to API v1.2.8

Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>
fixed missing API functions declaration in API ; fixed a typo in APIHandler.

Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>
added full comments to the new API functions.

Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>
Merge pull request #1693 from guyzmo/api_extension

extension of the API (versioned 1.2.8)