Add NSCopying support to FEMMapping

[k06a]

Also change a few FEMMapping readonly properties to readwrite

Discussed here: #73

[dimazen]

I have changed base branch to master, because develop is no longer in use.

[dimazen]

It also looks like a shallow copy and may cause unexpected behavior when Mapping A gets copied and later attributes updates somehow. Would you be mind to add copying support to Attribute and Relationship as well? Sorry for not mentioning it before.

[codecov-io]

Current coverage is 92.67% (diff: 0.00%)

Merging #74 into master will decrease coverage by 0.93%

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- Misses          196        227    +31   
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[dimazen]

After two minor fixes look good to merge.

[dimazen]

I would suggest to use

if (self.entityName != nil) { 
     = [self.class ....] initWithEntityName:];
else { 
     = [self.class ....] initWithObjectClass:];
}

[dimazen]

Also assignment of objectClass below needs to be removed in favor of using initializers

[k06a]

Replaced with conditional initialiser call

[dimazen]

We also need to override setters and nil-out counterpart. Otherwise you can set both objectClass and entityName which might look strange

- (void)setEntityName:(NSString *)entityName 
{
    _objectClass = nil;
    _entityName = [entityName copy];
}

- (void)setObjectClass:(Class)objetClass
{
    _objectClass = objetClass;
    _entityName = nil;
}

[k06a]

Done, and improved for case setting nil to already nilled property.

[dimazen]

I will rollout 1.1.1 tomorrow with added documentation from #69

[k06a]

@dimazen Thanks!