Update README.md : Travis status

I like that :)
Merge pull request #42 from lclc/patch-1

Update README.md : Travis status
Use Co-Z arithmetic for WINDOW_A precomputations

- Selected Co-Z formulas from "Scalar Multiplication on Weierstraß Elliptic Curves from Co-Z Arithmetic" (Goundar, Joye, et. al.) added as group methods with new type sep256k1_coz_t.
- Co-Z methods used for point precomputations in secp256k1_ecmult.
- WINDOW_A size increased to 6 since the precomputation is much faster per-point.
- DBLU cost: 3M+4S, ZADDU cost: 5M+2S.
- From 2.4% to 3.8% faster 'bench' results, depending on configuration.