Fix #12 remove rustc_serialize dependency

[g-k]

Requires rust with custom attributes >=1.18

[g-k]

Windows failure:

error: failed to run custom build command for `libz-sys v1.0.13`
process didn't exit successfully: `C:\projects\cargo-show\target\debug\build\libz-sys-b5f340344e0be7d0\build-script-build` (exit code: 1)
--- stdout
running: "C:\\Program Files (x86)\\Microsoft Visual Studio 14.0\\VC\\bin\\nmake.exe" "/nologo" "/f" "C:\\projects\\cargo-show\\target\\debug\\build\\libz-sys-a8ee7250d620a686\\out\\build/win32/Makefile.msc" "zlib.lib"
command did not execute successfully, got: exit code: 2
--- stderr
Microsoft (R) Program Maintenance Utility Version 14.00.24210.0
Copyright (C) Microsoft Corporation.  All rights reserved.
NMAKE : fatal error U1065: invalid option '-'
Stop.
Build failed, waiting for other jobs to finish...

looks related to rust-lang/cargo#4156