Upgrade some of the dependencies while at it.

Also enable -unchecked warnings in SBT build file.

Fix deprecations when compiled with Scala 2.8.1

tabulate used if indexed used by function and fill otherwise.

thus clean up the various singletons (RDDCache, MapOutputTracker, etc).
This isn't perfect yet (ideally we shouldn't use singleton objects at
all) but we can fix that later.

Bagel: Large-scale graph processing on Spark

Merging in Ankur's code for a cache that spills to disk

Refactored out the agg() and comp() methods from Pregel.run.

Defined an implicit conversion to allow applications that don't use
aggregators to avoid including a null argument for the result of the
aggregator in the compute function.

Note: This test suite currently fails for the same reason that the
Spark Core test suite fails: Spark currently seems to have a bug where
any test after the first one fails.

Also renamed the property spark.DiskSpillingCache.cacheDir to spark.diskSpillingCache.cacheDir in order to follow conventions.

DiskSpillingCache is a BoundedMemoryCache that spills entries to disk
when it runs out of space. Currently the implementation is very
simple. In particular, it's missing the following features:

- Error handling for disk I/O, including checking of disk space levels
- Bringing an entry back into memory after fetching it from disk

In addition, here are some features that aren't critical but should be
implemented soon:

- Spilling based on a user-set priority in addition to LRU
- Caching into a subdirectory of spark.DiskSpillingCache.cacheDir
  rather than the root directory