Rewrite calls of meta_exec with new structure

- Pass a table to meta_exec containing parameters, function to be
   called, scope string and requirements to check
 - Add basic ownership and contextid requirement checks in meta_exec
   when needed
 - Update README.md
 - Move assertion tools to assertions.lua

README.md

@@ -2,7 +2,7 @@ Minetest mod metatools
 ######################
 
 A mod inspired by mgl512's itemframe issue
-Version : 1.2
+Version : 1.2.1
 
 # Authors
  - LeMagnesium / Mg / ElectronLibre : Source code writer
@@ -29,21 +29,7 @@ allowing to see/edit metadatas
  - palige who agreed to test the mod for its first release, and contributed to the last version
 
 # Command tutorial
-
- - help										=> Get help
- - version									=> Get version
- - open (x,y,z)	_mode_							=> Open the node to manipulate at pos (x,y,z) with mode _mode_ (default is 'fields')
- - show 									=> Show fields/path list at actual position
- - enter _path_								=> Enter next stratum through _path_
- - leave										=> Leave current field and go back to previous stratum
- - set _name_ _value_						=> Set metadata _name_ to _value_ (create it if it doesn't exist)
- - itemstack								=> Manipulate itemstacks in Node/inventory/\*/
-	- erase _name_							=> Erase itemstack at field name
-	- write _name_ _itemstring_ [_count_]	=> Set itemstack in field _name_ with item _itemstring_ and count _count_. Default count is one, 0 not handled.
- - list
- 	- init _name_ _size_			=> Create a list of size _size_ named _name_
-	- delete _name_				=> Delete any list that could be called _name_
- - close									=> Close node
+ - Soon to come, please refer to /meta help until then
 
  Node metadatas look like this :