Make Plume compile on release (#365)

* Remove use of String for body parameters

Create SignedJson and implement FromData for it

* Make Travis test on release

* Remove warning when installing and fix coverage

.codecov.yml

@@ -0,0 +1,26 @@
+codecov:
+  notify:
+    require_ci_to_pass: yes
+
+coverage:
+  precision: 2
+  round: down
+  range: "70...100"
+
+  status:
+    project: no
+    patch: no
+    changes: no
+
+parsers:
+  gcov:
+    branch_detection:
+      conditional: yes
+      loop: yes
+      method: no
+      macro: no
+
+comment:
+  layout: "header, diff"
+  behavior: default
+  require_changes: no