Fix callback handle pointers

[mtrmac]

Use pointers to the relevant data structures which live as long as the callbacks can be called, not pointers to local variables which go away immediately.

(This is noticeable e.g. on CentOS 7, already in go test. It is quite suprising that this has worked just fine on other platforms so far. :) )

[proglottis]

Whoops, merged this and then realised it breaks the 1.6 cgo checks.

panic: runtime error: cgo argument has Go pointer to Go pointer [recovered]
    panic: runtime error: cgo argument has Go pointer to Go pointer

goroutine 22 [running]:
panic(0x416f680, 0xc82007c7e0)
    /usr/local/Cellar/go/1.6.1/libexec/src/runtime/panic.go:464 +0x3e6
testing.tRunner.func1(0xc8200a41b0)
    /usr/local/Cellar/go/1.6.1/libexec/src/testing/testing.go:467 +0x192
panic(0x416f680, 0xc82007c7e0)
    /usr/local/Cellar/go/1.6.1/libexec/src/runtime/panic.go:426 +0x4e9
github.com/proglottis/gpgme._cgoCheckPointer0(0x4135aa0, 0xc8200a6270, 0xc82007c7d0, 0x1, 0x1, 0x4124b60)
    ??:0 +0x4d
github.com/proglottis/gpgme.NewDataReader(0x50445d8, 0xc820060330, 0x429a818, 0x0, 0x0)
    /Users/james/Development/go/src/github.com/proglottis/gpgme/data.go:109 +0x3c8
github.com/proglottis/gpgme.TestDataNewDataReader(0xc8200a41b0)
    /Users/james/Development/go/src/github.com/proglottis/gpgme/data_test.go:70 +0xfc
testing.tRunner(0xc8200a41b0, 0x427e4e8)
    /usr/local/Cellar/go/1.6.1/libexec/src/testing/testing.go:473 +0x98
created by testing.RunTests
    /usr/local/Cellar/go/1.6.1/libexec/src/testing/testing.go:582 +0x892
exit status 2
FAIL    github.com/proglottis/gpgme 0.017s

Looking into it now.

[mtrmac]

Sorry for not noticing that, and thanks for the fix. But see #7.