Add bench_sign tool
Add verification to 32bit field

- implement _fe_verify for 10x26
- fe is normalized after _fe_clear
- a few corresponding changes in 64bit field
Better .gitignore for bench binaries
Merge pull request #83

7d681ac Add verification to 32bit field (Peter Dettman)
Merge pull request #82

8f9a307 Better .gitignore for bench binaries (Pieter Wuille)
fa5c13f Add bench_sign tool (Pieter Wuille)
Fix build for 64bit field under OSX

- caused by https://github.com/bitcoin/secp256k1/commit/8881212ebc43e67052ec06dec8beb459769fbab7
- OSX's ar tool doesn't work for empty archives ("ar: no archive members specified")
- introduce COMMON_LIB variable; leave empty when not using asm