Add include guard and namespace isolation

[v1993]

It isn't good to fill glogal namespace, so it is better to put all function in naespace like easy_encryption.

Also, it is very important to protect headers with include guards (or #pragma once, or even better both).

[v1993]

Refer to this commit https://github.com/clear-m/easy-encryption/commit/b65989f2d2e05c6968d9534cc7898d6389de56ff. It in fact include all requested changes.

[philipperemy]

Could you submit a PR? :)

[v1993]

I'll do it when will be at home, on next week. Please, don't close it yet.

[philipperemy]

Sure thanks a lot @v1993 )