add include of string.h to fix build issue on ubuntu

Was getting the following when building on ubuntu, though not on OS X.  Seems to be fixed by including string.h, which should be universally available.

```
> pripub@0.2.4 install /home/femto113/node_modules/pripub
> node-gyp rebuild

make: Entering directory `/home/femto113/node_modules/bcrypt/build'
  CXX(target) Release/obj.target/bcrypt_lib/src/blowfish.o
make: Entering directory `/home/femto113/node_modules/pripub/build'
  CXX(target) Release/obj.target/pripub/src/pripub.o
../src/pripub.cc: In static member function ‘static v8::Handle<v8::Value> pripub::PriPub::SetKeyPassword(const v8::Arguments&)’:
../src/pripub.cc:193:47: error: ‘memcpy’ was not declared in this scope
../src/pripub.cc: In static member function ‘static int pripub::PriPub::PasswordCallback(char*, int, int, void*)’:
../src/pripub.cc:209:46: error: ‘memcpy’ was not declared in this scope
make: *** [Release/obj.target/pripub/src/pripub.o] Error 1
make: Leaving directory `/home/femto113/node_modules/pripub/build'
```