Add support for app engine

[miko-code]

Hi , I
want to use this library on the app engine but i got some issues when i'm trying to submit an error i think it's because the App engine urlfetch issue ?

Any suggestion how to solve it ?

panic: runtime error: invalid memory address or nil pointer dereference

goroutine 14 [running]:
github.com/MindscapeHQ/raygun4go.(_Client).submit(0xc010428ab0, 0xc0104b7800, 0x14, 0x130c010, 0xd, 0x0, 0x0, 0xc0104dc940, 0x40, 0xc010536428, ...)
github.com/MindscapeHQ/raygun4go/raygun4go.go:190 +0xb79
github.com/MindscapeHQ/raygun4go.(_Client).HandleError(0xc010428ab0)
github.com/MindscapeHQ/raygun4go/raygun4go.go:148 +0x3e2
github.com/MindscapeHQ/raygun4go.(_Client).submit(0xc010428ab0, 0xc0104b6700, 0x14, 0x130c010, 0xd, 0x0, 0x0, 0x1331f30, 0x10, 0xc010430ea8, ...)
github.com/MindscapeHQ/raygun4go/raygun4go.go:190 +0xb79
github.com/MindscapeHQ/raygun4go.(_Client).CreateError(0xc010428ab0, 0x1331f30, 0x10)
github.com/MindscapeHQ/raygun4go/raygun4go.go:161 +0x1d8
dumpsterCloud/dumpster.(_DumpsterAPI).GetUserInfo(0x16572e8, 0xc0104c45b0, 0x0, 0x0, 0x0)
dumpsterCloud/dumpster/api.go:81 +0x2e7
reflect.Value.call(0x11affa0, 0x12741a0, 0x13, 0x12d2140, 0x4, 0xc0104db4e8, 0x2, 0x2, 0x0, 0x0, ...)
/tmp/appengine/go/src/reflect/value.go:419 +0x10fd
reflect.Value.Call(0x11affa0, 0x12741a0, 0x13, 0xc0104db4e8, 0x2, 0x2, 0x0, 0x0, 0x0)
/tmp/appengine/go/src/reflect/value.go:296 +0xe3
github.com/GoogleCloudPlatform/go-endpoints/endpoints.(_Server).ServeHTTP(0xc01049a760, 0x7f2ca12d8600, 0xc01042f680, 0xc0104c45b0)
github.com/GoogleCloudPlatform/go-endpoints/endpoints/server.go:155 +0xc2f
net/http.(_ServeMux).ServeHTTP(0xc010464720, 0x7f2ca12d8600, 0xc01042f680, 0xc0104c45b0)
go/src/net/http/server.go:1541 +0x1b4
appengine_internal.executeRequestSafely(0xc01042f680, 0xc0104c45b0)
go/src/appengine_internal/api_prod.go:288 +0xb7
appengine_internal.(_server).HandleRequest(0x164c100, 0xc010400ea0, 0xc0104fa180, 0xc010538340, 0x0, 0x0)
go/src/appengine_internal/api_prod.go:222 +0x102b
reflect.Value.call(0x1229060, 0x164c100, 0x113, 0x12d2140, 0x4, 0xc0104dbf78, 0x3, 0x3, 0x0, 0x0, ...)
/tmp/appengine/go/src/reflect/value.go:419 +0x10fd
reflect.Value.Call(0x1229060, 0x164c100, 0x113, 0xc0104dbf78, 0x