Add Go language bindings.

Squashed commit of the following:

commit b604248e60dbe08a2056c3cd046b111b518c780f
Author: Casey Marshall <cmars@cmarstech.com>
Date:   Fri May 30 20:48:56 2014 -0500

    Add third-party discharge macaroon test

    Also fix Verifier.Verify arg type and VerifyExact test.

commit f051306a6ff28ba9789197ae7a1b3eb80d59a23c
Author: Casey Marshall <cmars@cmarstech.com>
Date:   Fri May 30 17:58:51 2014 -0500

    Verifier test, satisfy general (callbacks), README

commit 28de71ad9155553a9aec972a499494cd46404cd7
Author: Casey Marshall <cmars@cmarstech.com>
Date:   Fri May 30 11:58:03 2014 -0500

    Remove unused #include.

commit 6a22e640bdd25600f18810d0990574ed2bbc80b4
Author: Casey Marshall <cmars@cmarstech.com>
Date:   Fri May 30 11:38:29 2014 -0500

    Finish API parity with python Macaroon object.

commit 9ab03132a6fee06d89f6dcf06f0f7524cf9acd51
Author: Casey Marshall <cmars@cmarstech.com>
Date:   Fri May 30 11:32:50 2014 -0500

    Switch gocheck dependencies to gopkg.in/check.v1

commit 00aae0e399afe424061b0f1f4383173c10105bef
Author: Casey Marshall <cmars@cmarstech.com>
Date:   Thu May 29 21:59:53 2014 -0500

    Add Verifier.

commit 357c832ed7f727bdf269465b12c29daf73aac20e
Author: Casey Marshall <cmars@cmarstech.com>
Date:   Thu May 29 21:42:06 2014 -0500

    Add serialization, getters, inspect.

commit 4abdefc0594c764a8b0049177fe2ea42ea3d9216
Author: Casey Marshall <cmars@cmarstech.com>
Date:   Thu May 29 19:35:51 2014 -0500

    Call Destroy what it is.

commit c05a838863ab37da159f53e7ba643da092c58b96
Author: Casey Marshall <cmars@cmarstech.com>
Date:   Thu May 29 19:28:53 2014 -0500

    Simplify C strings & byte slices.

    Add utility functions from sqlite driver.

commit acdf3fa35d2a50daa376d8c4c27fa0d6f1bdd4bc
Author: Casey Marshall <cmars@cmarstech.com>
Date:   Thu May 29 13:50:13 2014 -0500

    Started on cgo bindings.