refactor coxpcall dependency (#41)

* alter global pcall & xpcall instead of their local copies

like ringer.lua & xavante.lua

* remove duplicated coxpcall stuff

all is already done in wsapi.common
see https://github.com/keplerproject/wsapi/blob/master/src/wsapi/common.lua#L21-L25

`coxpcall` (like `rings`) are not longer a direct dependencies of `wsapi-fcgi` & `wsapi-xavante`.
So, the `rockspec` could be updated like this:

```diff
wsapi:
-dependencies = { "luafilesystem >= 1.6.2" }
+dependencies = { "luafilesystem >= 1.6.2", "rings >= 1.3.0", "coxpcall >= 1.14" }

wsapi-fcgi:
-dependencies = { "wsapi >= 1.6.1", "rings >= 1.3.0", "coxpcall >= 1.14" }
+dependencies = { "wsapi >= 1.6.x" }

wsapi-xavante:
-dependencies = { "wsapi >= 1.6.1", "rings >= 1.3.0", "coxpcall >= 1.14", "xavante >= 2.3.0" }
+dependencies = { "wsapi >= 1.6.x", "xavante >= 2.3.0" }
```

* coxpcall is useless with LuaJIT

use same logic as in copas
see https://github.com/keplerproject/copas/blob/master/src/copas.lua#L28-L30

* update history/changelog

Author: fperrad

doc/us/index.html

@@ -131,6 +131,12 @@ <h2>Latest Sources and Bug Tracker</h2>
 
 <h2>History</h2>
 
+<p><strong>WSAPI</strong> [unreleased]</p>
+
+<ul>
+    <li>Refactor coxpcall dependency (Francois Perrad)</li>
+</ul>
+
 <p><strong>WSAPI 1.6.1</strong> [21/Mar/2014]</p>
 
 <ul>

src/wsapi/common.lua

@@ -15,15 +15,15 @@ local debug = require "debug"
 local wsapi = require "wsapi"
 local lfs = require "lfs"
 
-local tostring, tonumber, pairs, ipairs, error, type, pcall, xpcall, setmetatable, dofile, rawget, rawset, assert, loadfile =
-  tostring, tonumber, pairs, ipairs, error, type, pcall, xpcall, setmetatable, dofile, rawget, rawset, assert, loadfile
-
-if _VERSION < "Lua 5.2" then
+if _VERSION=="Lua 5.1" and not jit then     -- obsolete: only for Lua 5.1 compatibility
 	local coxpcall = require "coxpcall"
 	pcall = coxpcall.pcall
 	xpcall = coxpcall.xpcall
 end
 
+local tostring, tonumber, pairs, ipairs, error, type, pcall, xpcall, setmetatable, dofile, rawget, rawset, assert, loadfile =
+  tostring, tonumber, pairs, ipairs, error, type, pcall, xpcall, setmetatable, dofile, rawget, rawset, assert, loadfile
+
 local package = package
 
 local _, ringer = pcall(require, "wsapi.ringer")

src/wsapi/ringer.lua

@@ -24,9 +24,6 @@ local init = [==[
     _, package.cpath = remotedostring("return package.cpath")
   end
   local common = require"wsapi.common"
-  local coxpcall = require "coxpcall"
-  pcall = coxpcall.pcall
-  xpcall = coxpcall.xpcall
   
   local wsapi_error = {
        write = function (self, err)

src/wsapi/xavante.lua

@@ -6,11 +6,6 @@
 --
 -----------------------------------------------------------------------------
 
-local coxpcall = require "coxpcall"
-
-pcall = coxpcall.pcall
-xpcall = coxpcall.xpcall
-
 local common = require"wsapi.common"
 
 local _M = {}