Add SDPX licence identifier to source files

[aldas]

Add SDPX licence identifier to source files. https://spdx.org/licenses/

There are cases when Echo code has been copied to other projects and due to the fact that we do not have licence at the start of file with copyright line and even with full MIT message at the beginning of the file without copyright statement the information is last that Echo was the original source. Yes, MIT allows copying but licence must be preserved. SPDX tags are shorter way to achieve what I am trying to do here - when code is copied it is easy to understand that Echo was the source (removing licence notes would be against the licences to my understanding).

so every go file gets

// SPDX-License-Identifier: MIT
// SPDX-FileCopyrightText: © 2017 LabStack and Echo contributors

Note: copyright line number is " year of publication " and does not need to be updated

[codecov]

Codecov Report

Patch coverage: 77.16% and project coverage change: +4.93 🎉

Comparison is base (b6855c2) 58.84% compared to head (abf5292) 63.77%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage   58.84%   63.77%   +4.93%     
==========================================
  Files           8        9       +1     
  Lines         690      944     +254     
==========================================
+ Hits          406      602     +196     
- Misses        260      302      +42     
- Partials       24       40      +16     

Impacted Files	Coverage Δ
casbin/casbin.go	89.13% <ø> (ø)
jaegertracing/jaegertracing.go	56.37% <ø> (ø)
jaegertracing/response_dumper.go	100.00% <ø> (ø)
pprof/pprof.go	100.00% <ø> (ø)
prometheus/prometheus.go	49.15% <ø> (ø)
session/session.go	100.00% <ø> (ø)
zipkintracing/response_writer.go	21.27% <ø> (ø)
zipkintracing/tracing.go	63.73% <ø> (ø)
echoprometheus/prometheus.go	77.16% <77.16%> (ø)

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