Signatures

Changes from https://github.com/devi/tweetnacl-js/tree/sha512 + new tests for signatures.
Signature test signs a random message in JS, then signs the same message with a C program using the latest tweetnacl.c, then compares the result and tries to open it.

To run sign test,

```
 cd tests
 make test
```

Currently sign test doesn't pass fully, e.g.:

```
Test #28 (Message length: 40)
sign - OK
open - OK

Test #29 (Message length: 40)
! signatures don't match
JS:  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAg5/uin4OUr0A3BGf1d6EjsjCeR4mWMOULHTUBe6hEAA== 
C :  0/NJ+frDasaDY8Y1PH/MPQRCvS/bMND9z/GwYOCOJKZ4q1C8/IWjhVNqguenzXuWahVePtrjMQZVjldDqeyUBw==
open - OK

Test #30 (Message length: 40)
sign - OK
open - OK

Test #31 (Message length: 44)
sign - OK
open - OK

Test #32 (Message length: 44)
sign - OK
open - OK

Test #33 (Message length: 44)
sign - OK
open - OK

Test #34 (Message length: 48)
! signatures don't match
JS:  VFrc0cgwtkAED6wgjrGtzYGlWhR+53f8sfzYGLiuP5PZLrCs/S9YWe/d5F4HJ+3BEuGIsaC8YgfAW0jcba3RAA== 
C :  VFrc0cgwtkAED6wgjrGtzYGlWhR+53f8sfzYGLiuP5OZ5spi8J982VDamORfRRiSOfC3/dOGZkcCbFSmVmCpAw==
! verification failed
```