Add itersubjects to Storage class

[ixjlyons]

It's pretty common to iterate over subjects in post-experiment analysis and that's kind of clunky at the moment:

storage = Storage('data')
for subj in storage.subject_ids:
    storage.subject_id = subj
    reader = storage.require_task('sometask')
    ...

Better:

storage = Storage('data')
for subj in storage.itersubjects():
    reader = storage.require_task('sometask')
    ...

It'd also be pretty rad if you could optionally pass a task to get the reader object instead:

storage = Storage('data')
for reader in storage.itersubjects(task='sometask'):
   ...

Should be pretty straightforward to implement.