Creation of a common structure IDL

* Created common.avdl
* Added error GAException type
* Moved GAKeyValue to common.avdl
Merge branch 'common_structure_idl' into alt_method_definitions
removed GAKeyValue from reads.avdl
Merge branch 'common_structure_idl' into alt_method_definitions

* needed to remove the GAKeyValue from reads.avdl and re-merge
added comment to RPC method to map to REST POST request /via @cassiedoll