Add Support for Tiered Pricing Plans

[jksimoniii]

What's this PR do?

• Models Tiers as they appear in Stripe
• Accounts for tiers during the sync_plans command
• Implements Plan.calculate_total_amount
• Update Subscription to use Plan.calculate_total_amount

Any background context you want to provide?

Stripe Tiered Pricing Docs

What ticket or issue # does this fix?

Closes https://github.com/pinax/pinax-stripe/issues/628

Definition of Done (check if considered and/or addressed):

• [x] Are all backwards incompatible changes documented in this PR?
• [x] Have all new dependencies been documented in this PR?
• [ ] Has the appropriate documentation been updated (if applicable)?
• [x] Have you written tests to prove this change works (if applicable)?

[codecov[bot]]

Codecov Report

Merging #629 into master will increase coverage by 0.02%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #629      +/-   ##
==========================================
+ Coverage   99.21%   99.23%   +0.02%     
==========================================
  Files          33       33              
  Lines        1911     1967      +56     
  Branches      175      185      +10     
==========================================
+ Hits         1896     1952      +56     
  Misses          7        7              
  Partials        8        8

Flag	Coverage Δ
#py27dj110	98.93% <100%> (+0.03%)	:arrow_up:
#py27dj111	98.93% <100%> (+0.03%)	:arrow_up:
#py27dj18	99.18% <100%> (+0.02%)	:arrow_up:
#py34dj110	98.93% <100%> (+0.03%)	:arrow_up:
#py34dj111	98.93% <100%> (+0.03%)	:arrow_up:
#py34dj18	99.18% <100%> (+0.02%)	:arrow_up:
#py34dj20	98.93% <100%> (+0.03%)	:arrow_up:
#py35dj110	98.93% <100%> (+0.03%)	:arrow_up:
#py35dj111	98.93% <100%> (+0.03%)	:arrow_up:
#py35dj18	99.18% <100%> (+0.02%)	:arrow_up:
#py35dj20	98.93% <100%> (+0.03%)	:arrow_up:
#py36dj111	98.93% <100%> (+0.03%)	:arrow_up:
#py36dj20	98.93% <100%> (+0.03%)	:arrow_up:
#py36dj20psql	98.93% <100%> (+0.03%)	:arrow_up:

Impacted Files	Coverage Δ
pinax/stripe/utils.py	100% <100%> (ø)	:arrow_up:
pinax/stripe/models.py	100% <100%> (ø)	:arrow_up:
pinax/stripe/managers.py	100% <100%> (ø)	:arrow_up:
pinax/stripe/actions/plans.py	100% <100%> (ø)	:arrow_up:

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