Implement Database::backup()

[os12]

• it is possible to easily backup/restore a live DB via the
  sqlite3_backup_init() API
  • https://www.sqlite.org/backup.html
  • https://www.sqlite.org/c3ref/backup_finish.html#sqlite3backupinit

[coveralls]

Coverage increased (+0.08%) to 96.586% when pulling a58ba7d on os12:master into 3ba20a3 on SRombauts:master.

[SRombauts]

Thanks a lot, I really like that you are making all these improvements! This one is even something I had implemented locally for one project, but never put int this wrapper.

I would suggest two changes though:

1. I usually try to stick with the underlying SQLite API name, so in this case it should be simply called "backup()"
2. The second parameter should be an enum, so it would be so much clearer at the call site without comments

If you agree to these I'll let you make the changes, but else QI am willing to merge this like that and improve on later myself

[os12]

I took the name and the API verbatim from the sample... but sure, tweaked the names as per your suggestion. That took about a minute :)

[SRombauts]

Thank you for your work, once again very appreciated!

I merged it into master and pushed a compilation fix for C++98, needed until we can start a new C+11 (or C++14) branch.

Cheers,
Sébastien

[os12]

Cool! Thanks for the fix Sébastien!

Now I need to decide whether I should rebase :)