Allow configuration of lib and include path through 'leveldb' name

Hi,

On Windows pretty painful to generate a Makefile which contains the correct path of leveldb headers without this option, bacause no 'ususal' path like on Linux. 
This change allow to pass extra path to mkmf, e.g.:
```
gem install leveldb-native -- --with-leveldb-dir=/g/tmp/leveldb-mingw-master
gem install leveldb-native -- --with-leveldb-lib=/g/tmp/leveldb-mingw-master/lib
gem install leveldb-native -- --with-leveldb-include=/g/tmp/leveldb-mingw-master/include
```

The same works with `rake compile`.