added support for <app>/limits/boot_timeout

[marcelloromani]

Addresses #100

[coveralls]

Coverage decreased (-0.5%) to 47.823% when pulling 5f1ea71 on smartpension:feature/support_boot_timeout into e3dc9c2 on martyzz1:master.

[codecov-commenter]

Codecov Report

Merging #101 (18030dc) into master (3f4161c) will increase coverage by 1.04%.
The diff coverage is 32.55%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   48.62%   49.67%   +1.04%     
==========================================
  Files          27       29       +2     
  Lines        1240     1391     +151     
  Branches      114      120       +6     
==========================================
+ Hits          603      691      +88     
- Misses        628      691      +63     
  Partials        9        9              

Impacted Files	Coverage Δ
heroku3/models/app.py	31.39% <24.32%> (-1.08%)	⬇️
heroku3/models/__init__.py	67.59% <83.33%> (ø)
heroku3/__init__.py	100.00% <0.00%> (ø)

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