Support additional JWT algorithms

[mmacai]

• added support to JWT verification to use RS256 algorithm
• now support HS256, RS256, should be easily extendible in future for other algorithms
• covered by tests
• configurable by env var JWT_ALG - defaults to HS256
• bumped up Joken version
• if using RS256 expects PEM key as a JWT_SECRET_KEY

closes #84

[kevinbader]

Looks good!

With Elixir 1.7.2 I get

warning: variable "msg" does not exist and is being expanded to "msg()", please use parentheses to remove the ambiguity or change the variable name
  lib/rig_inbound_gateway_web/presence/channel.ex:170


== Compilation error in file lib/rig_inbound_gateway_web/presence/channel.ex ==
** (CompileError) lib/rig_inbound_gateway_web/presence/channel.ex:170: undefined function msg/0
    (stdlib) lists.erl:1338: :lists.foreach/2
    (stdlib) erl_eval.erl:677: :erl_eval.do_apply/6

This is because of the side effect (assignment) happening within a log statement that gets stripped by the compiler according to the log level in use. Anyway, this is solved in #80 (https://github.com/Accenture/reactive-interaction-gateway/pull/80/files#diff-44e879ef2af4eddd079e66e7059d35d2R169), so let's simply merge this one afterwards.