skip md5 when it is not sended

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

use datetime.datetime.utcnow() to create timestamps

closes #27

The following methods now exist for /file endpoints:
GET /<level>/<id>/file/<filename>
    authenticated file download
GET /<level>/<id>/file/<filename>?ticket
    authenticated download ticket request
GET /<level>/<id>/file/<filename>?ticket=<ticket>
    unauthenticated (ticketed) file download
DELETE /<level>/<id>/file/<filename>
    delete file
PUT /<level>/<id>/file/<filename>
    update file metadata (not yet implemented)
POST /<level>/<id>/file/<filename>
    upload file as "body file"
POST /<level>/<id>/file
    upload file as multipart/form-data