Modified the coin validity checks to ensure that coins are verified immediately
upon deserialization. Added a 'valid' flag to each PublicCoin object, and
an isValid() method to check it. Also changed the Accumulator to check this flag
rather than repeating the validity test unnecessarily.
Finally: wrote a new unit test to test against invalid (non-prime) coins.
Switched all invalid_argument exceptions to be ZerocoinExceptions.
This means we can properly catch them all with a single catch {} block.
Merge branch 'dev' of github.com:Zerocoin/libzerocoin into dev
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'