Add new baseQueue and underlying priority queue code and tests.

baseQueue will be embedded in all range queue implementations.
adjust scanner timing test to be more permissive
Implementation of rangeIterator for store.
Update scanner.go
Address feedback, add unittest, fixed bug in Store.RemoveRange
Address feedback
Merge pull request #218 from cockroachdb/spencerkimball/store-range-iterator

Implementation of rangeIterator for store.
Merge pull request #217 from cockroachdb/spencerkimball/watch-queue

Add new baseQueue and underlying priority queue code and tests.
README.md: Update to remove RocksDB confusion.

Previously we asked that people install the reqs for RocksDB, but that caused linking issues with bootstrap.sh. Removing that instruction for now.
Log instead of panicking when ProposeRaftCommand called after shutdown.

When a test that uses goroutines fails, these goroutines may continue
to try to use the store after it is shut down. The panic occurs during a
subsequent test function and obscures the true cause of the failure.

This problem appears to occur most often with TestRangeIdempotence,
so I've increased its timeout.

Fixes #222.
Merge pull request #223 from bdarnell/fix-panic

Log instead of panicking when ProposeRaftCommand called after shutdown.