Explicitly install gcc-4.7 in Dockerfile.

The "golang" base image has been changed to use a more recent version of
debian, which does not install gcc-4.7 by default.
Merge pull request #129 from bdarnell/docker

Explicitly install gcc-4.7 in Dockerfile.
Provide our own C interface to rocksdb.

Stop using the rocksdb C interface and instead provide our own C
interface to rocksdb. This makes it cleaner to implement various bits of
cockroach-specific functionality in C++ such as the compaction filter
logic and merge operator.
Fleshed out the documentation in db.h.