blob_t implementation

[aminroosta]

@zauguin @Killili Please review this PR :-)
There might be a better approach, so far i've came up with this.

#122

[zauguin]

Except for the sqlite_config, this is with .vec the only place the interface relies on public data members. What do you think about using sth. like operator *, because blob_t is a wrapper and we want to access the underlying object?

[Killili]

I like it, it makes a lot more sense than just having vector<> as an option for operator<< to make blobs.
What this implies is "use a blob to store x" and x is a vector because thats all we can handle for now. But blob() can be expanded on in the future to handle all kinds of stuff.
So i do not like that it is an interface for vector only but i like it for the expend-ability later on. It sets an example for how we might handle custom data types later on or even explicitly choosing the SQL Type like:

auto val = get_something_numerical_maybe_float_or_int();
db << "insert ...." << val; // might end up numeric or real
db << "insert ...." << real(val); // will always be a real or error if for example its out of range