First shot at Amazon Glacier support.

see #352 and discussion there

short summary:
* working: configuration, storing in Glacier files not larger than 10GB (constant chunk size)
* code and specs based heavily off S3 implementation
* #nearest_power_of_two method and file-size checking ready for implementing chunk size calculation
* not implemented: removing archives (used for cycling/keep)

Stuff that would be nice to have:
* Removing archives for cycling/keep. Non-trivial, but easier given the current Backup structure (Backup's /data/ directory with .yml storing performed backups' metadata -- we can store glacier archive id there for easy removal)
* dynamic chunk-size calculation; the methods are there, I'll add it soon (just wanted to push it out the door for other people to test)

Specs are green, but with such gems nothing beats real-world testing.
It works for me (as in: tested on real backup scenarios), but testing will be HIGHLY appreciated.
Example config can be easily adapted from existing S3 config, just remember to create the Vault first:
```
  store_with Glacier do |glacier|
    glacier.access_key_id      = 'key'
    glacier.secret_access_key  = 'secret'
    glacier.vault              = 'testvault'
    glacier.region             = 'us-east-1'
    glacier.keep               = 3
  end
```