Improve rrd2whisper's handling of rrd files

Take into account the actual retentions in rrds and make whisper
archives similarly. Also allow selection of non-AVERAGE rrd archive
types
Merge pull request #19 from lambdafu/master

Fix precision error message
Revert "Replace use of dict for duplicate removal"

This reverts commit 48bcbbfc92546618d09843f38804769341eaaa61.

The changes introduced by 48bcbbf cause a "'str' object does not support
item assignment" exception; this patch restores the previous behavior
until a more permanent fix to the issue 48bcbbf attempts to address is
made.
Merge pull request #37 from ajablonski/master

Revert "Replace use of dict for duplicate removal"
Merge pull request #40 from graphite-project/0.9.x

Merge a few patches from master
fix typo
make uniform title underlines
fix missing periods in short descriptions
fix missing newline
Merge pull request #51 from esc/fix_readme

minor fixes to the readme
fix indentation to make whisper-merge work with more than one archive
Merge pull request #54 from ImmobilienScout24/master

fix indentation to make whisper-merge work with more than one archive
Remove py2.5 from travis as it's no longer supported

Also remove tox as it's not executed and now redundant
Merge pull request #55 from mleinart/remove_travisci_py25

Remove py2.5 from travis as it's no longer supported
Optionally specify fixed 'now' to fetch()

This is used by a corresponding graphite change to
ensure consistent data resolution selection over multiple
calls to fetch()
Merge pull request #56 from jcsp/consistent-now

Optionally specify fixed 'now' to fetch()
Merge branch 'master' of github.com:graphite-project/whisper into development

Merging graphite-projects before starting new development.