Second step in converting to C: field
Merge pull request #9 from sipa/cify

Second step in converting to C: field
Switch num/field to extern C; small fixes
Use variable-time fe_inv in group
Split 5x52 inner implementations
Source tree reorganization
Bugfix: secp256k1_fe_inv_var correct output
Remove num_builtin.h; it wasn't complete
Compile with -fPIC so that we can build shared libraries.
Add JNI wrapper for org.bitcoin.NativeSecp256k1 (bitcoinj)

This isnt as optimal as it could be, but its also a very simple API
which is nice.