Creation of a common structure IDL

* Created common.avdl
* Added error GAException type
* Moved GAKeyValue to common.avdl
removed GAKeyValue from reads.avdl
Merged github.com/ga4gh/schemas master branch

Conflicts:
	src/main/resources/avro/reads.avdl

Conflict was the GAKeyValue record which was moved to common.avdl
Merge pull request #58 from delagoya/common_structure_idl

Creation of a common structure IDL