Centralized HTTP error handling

• Centralized HTTP error handling http://labstack.github.io/echo/guide/#error-handling
  • Default handler function changed from func(*Context) error to func(*Context) *echo.HTTPError.
  • Most of the response helper functions now return *echo.HTTPError http://labstack.github.io/echo/guide/#response
• Fixed issues #46, #49 and #50.
• Added echo.HandlerFunc to the list of supported handlers.
• More documentation.