Rename JWTMiddleware to JwtMiddleware

[msrd0]

This brings our API in line with the Rust API Guidelines. Follow-Up for #534

The old name is now a type alias that I marked deprecated as of 0.6.1. We should then probably publish 0.6.1 straight away so that people using the middleware get the deprecation warning and have time to update their code before the next breaking release.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@16a893f). Click here to learn what that means.
The diff coverage is 83.82%.

❗ Current head e311902 differs from pull request most recent head 9316cef. Consider uploading reports for the commit 9316cef to get more accurate results

@@            Coverage Diff            @@
##             master     #537   +/-   ##
=========================================
  Coverage          ?   85.00%           
=========================================
  Files             ?      110           
  Lines             ?     5608           
  Branches          ?        0           
=========================================
  Hits              ?     4767           
  Misses            ?      841           
  Partials          ?        0           

Impacted Files	Coverage Δ
gotham/src/extractor/path.rs	66.66% <0.00%> (ø)
gotham/src/extractor/query_string.rs	66.66% <0.00%> (ø)
gotham/src/helpers/http/mod.rs	83.87% <ø> (ø)
gotham/src/helpers/timing.rs	0.00% <0.00%> (ø)
gotham/src/middleware/logger/mod.rs	0.00% <0.00%> (ø)
gotham/src/middleware/security/mod.rs	0.00% <0.00%> (ø)
gotham/src/middleware/timer/mod.rs	0.00% <0.00%> (ø)
gotham/src/pipeline/set.rs	100.00% <ø> (ø)
gotham/src/pipeline/single.rs	100.00% <ø> (ø)
gotham/src/router/builder/modify.rs	40.00% <ø> (ø)
... and 99 more

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