Allow SetWeak() for non-object persistent handles.

[bnoordhuis]

The definition of the SetWeak() template method constrained the type to
types with a GetAlignedPointerFromInternalField() method. In practice
that meant it only worked with T=v8::Object.

Lift that restriction so that e.g. T=v8::External now also works. Note
that SetWeak(WeakCallbackType::kInternalFields) for types without the
aforementioned method will result in a run-time assertion.

---

I wrote this over a year ago but I forgot to PR it. I don't need it now but I did want it at the time and it still seems useful so here goes.

[bnoordhuis]

CI failure is a node-gyp incompatibility with Node.js v0.8.28 (!):

gyp info it worked if it ends with ok
gyp info using node-gyp@3.6.3
gyp info using node@0.8.28 | darwin | x64
gyp info chdir test
gyp ERR! UNCAUGHT EXCEPTION 
gyp ERR! stack TypeError: Cannot read property 'prototype' of undefined
gyp ERR! stack     at Object.exports.inherits (util.js:538:43)
gyp ERR! stack     at Object.<anonymous> (/Users/travis/build/nodejs/nan/node_modules/request/node_modules/http-signature/node_modules/sshpk/lib/ed-compat.js:25:6)
gyp ERR! stack     at Module._compile (module.js:449:26)
gyp ERR! stack     at Object.Module._extensions..js (module.js:467:10)
gyp ERR! stack     at Module.load (module.js:356:32)
gyp ERR! stack     at Function.Module._load (module.js:312:12)
gyp ERR! stack     at Module.require (module.js:362:17)
gyp ERR! stack     at require (module.js:378:17)
gyp ERR! stack     at Object.<anonymous> (/Users/travis/build/nodejs/nan/node_modules/request/node_modules/http-signature/node_modules/sshpk/lib/private-key.js:17:16)
gyp ERR! stack     at Module._compile (module.js:449:26)
gyp ERR! System Darwin 17.4.0
gyp ERR! command "node" "/Users/travis/build/nodejs/nan/node_modules/.bin/node-gyp" "rebuild" "--directory" "test"
gyp ERR! cwd /Users/travis/build/nodejs/nan/test
gyp ERR! node -v v0.8.28
gyp ERR! node-gyp -v v3.6.3

[bnoordhuis]

Rebased onto master. CI should be passing now that #826 is merged.

[kkoopa]

LGTM