Closes #1

Nicer warning behavior for PersistentDict

See merge request !7

* Add a CacheCollision warning subclass.
* Track stack level so that warnings show user code line numbers.

Example:

Modifying line 287 of test_persistent_dict.py so that the warning
isn't captured by the test, we get this:

```
test_persistent_dict.py:287: CollisionWarning: pytools-test: key collision in cache at '/tmp/tmpq9p3i0b9' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
  pdict[key2]  # user code
```

The old behavior was:

```
/home/matt/src/pytools/pytools/persistent_dict.py:466: UserWarning: pytools-test: key collision in cache at '/tmp/tmpuk8js9jw' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
  % (self.identifier, self.container_dir))
```

Fix value used for testing store_if_not_present()

See merge request !6

Store if not present in persistent dicts

See merge request !5

PersistentDict cleanup; create WriteOncePersistentDict

See merge request inducer/pytools!4

Conflicts:
	test/test_persistent_dict.py

Major changes to PersistentDict:
  * Uses a per-file lock instead of a per-container-dir lock.
  * Removed info files from store().
  * Checks for cache collision on delete.
  * Added more tests.

This change also implements WriteOncePersistentDict, which has a
write-once policy.  WriteOncePersistentDict uses a writer lock but
allows for unlocked reads, which makes reads from disk faster.  It
also uses an in-memory LRU cache to speed up accesses.

the version read is for backwards compatibility.

This adds an optional in-memory LRU cache. To use the cache, you
supply a *in_mem_cache_size* parameter to the PersistentDict.

In order to properly support cache invalidation this also implements
version tracking of files. This change is backwards compatible with
existing persistent dict caches.

Nested saved persistent hashes

See merge request !3