fix avatar handling and create schema
filter permissions for containers in handlers

use tempdir custom module instead of tempfile
fail with 400 when ObjectIds are not valid
check existing schemas against a list at startup
add json schema draft 4 specification
give a more detailed error message for keys mismatch
remove upload method in core.py = dependencies (util.upload_ticket +
json schema)
remove search call + method = schema

closes #27

handle files with a length that is an exact multiple of blocksize

relates to scitran/sdm#37

There are now three ways to retrieve sessions:
- /groups/<gid>/sessions
- /projects/<pid>/sessions
- /sessions (returns all sessions; use with caution)

TODO: implement pagination