cpp: add ability to read w/o checksumming

Added overloaded versions of following functions:
    std::string read(...)
    int read(...)
    void read_hashed(...)
    std::string read_hashed(...)

Now they accept parameter of type
    enum eblob_read_flavour csum
which can be either:
    EBLOB_READ_CSUM
    EBLOB_READ_NOCSUM