Updated CMake to add 'make install' and 'make uninstall'

* Installs libzercoin.so to $PREFIX/lib
    
* Installs Zerocoin headers to $PREFIX/include/zerocoin
    
* Installs bignum headers to $PREFIX/include/zerocoin/bitcoin_bignum
    
* Installs pkg-config file for library name 'zerocoin'