Add equality operators to compare Enum with enumerated value

[gamante91]

The aim of this PR is to add the possibility to compare enums with values without the need to add + to explicitly promote them to types.

For example the following code is possible:

Channel red = Channel::Red;

if (red != Channel::Blue) {
}
if (red == Channel::Red) {
}
if (Channel::_from_string("Red) != Channel::Red) {
}

See unit tests for examples.

[gamante91]

@aantron this should close issue #39.

I don't know how to associate this PR with it.