Refactor package namespace according to new conventions

[nezhar]

Renames the package to drf-multitokenauth and the package namespace to drf_multitokenauth. It also includes an upgrade guide for the next major release wich will use the new namespace.

[codecov-commenter]

Codecov Report

Merging #29 (3d75184) into master (5aac628) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 3d75184 differs from pull request most recent head 4e3c71e. Consider uploading reports for the commit 4e3c71e to get more accurate results

@@           Coverage Diff           @@
##           master      #29   +/-   ##
=======================================
  Coverage   90.41%   90.41%           
=======================================
  Files          10       10           
  Lines         146      146           
=======================================
  Hits          132      132           
  Misses         14       14           

Impacted Files	Coverage Δ
drf_multitokenauth/migrations/0001_initial.py	100.00% <ø> (ø)
...auth/migrations/0002_rename_ip_address_20160426.py	100.00% <ø> (ø)
drf_multitokenauth/serializers.py	100.00% <ø> (ø)
drf_multitokenauth/signals.py	100.00% <ø> (ø)
drf_multitokenauth/admin.py	100.00% <100.00%> (ø)
drf_multitokenauth/coreauthentication.py	65.00% <100.00%> (ø)
drf_multitokenauth/migrations/0003_pk_migration.py	80.95% <100.00%> (ø)
drf_multitokenauth/models.py	96.29% <100.00%> (ø)
drf_multitokenauth/urls.py	100.00% <100.00%> (ø)
drf_multitokenauth/views.py	95.91% <100.00%> (ø)

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