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'
Merge pull request #7 from jmcorgan/cmake

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