Pluggable storage engines for carbon #210

	added db abstract class
	wired writer.py to abstract DB class
	got db plugin to work on write side
	added fetch func for webapp integration
	finished renaming DB to TSDB
	added dependency on graphite-data
	removed graphitedata dependency and prettied up the diff
	fixed bug with exists()
	removed unnecessary fetch function from WhisperDB
	added commented-out hooks to graphitedata
	added commented out plugin config to carbon.conf.example