Influx DB Export functionality

[aanthony1243]

Update backup command to support an 'export' functionality:

the original backup method will remain, though the database name will be required:

influxd backup -database -since /path/to/backup

where '-since' selects data to output based on the age of the files on disk. This is a more operational-style backup that gets all changes since that date.

Many customers want to use the timestamps in the database to control the backup/export. We add two new flags:

influxd backup -database -start -end /path/to/backup

In this case, each file on disk will be scanned and filtered so that only points inside the start/end date will be exported.

note: 'since' and 'start/end' are mutually exclusive. if both are specified, an error is returned and the usage will be printed.