Always fall back to Plist values + withDefault

* Always fall back to `LiquidVariables.plist` if a liquid package value isn't present.
* A new option `withDefault` for each get value method was added:

    -(NSNumber *)numberValueForVariable:(NSString *)variableName withDefault:(NSNumber *)defaultValue;
    -(NSDate *)dateValueForVariable:(NSString *)variableName withDefault:(NSDate *)defaultValue;
    -(UIColor *)colorValueForVariable:(NSString *)variableName withDefault:(UIColor *)defaultValue
    -(NSString *)stringValueForVariable:(NSString *)variableName withDefault:(NSString *)defaultValue
    -(NSInteger)intValueForVariable:(NSString *)variableName withDefault:(NSInteger)defaultValue;
    -(CGFloat)floatValueForVariable:(NSString *)variableName withDefault:(CGFloat)defaultValue;
    -(BOOL)boolValueForVariable:(NSString *)variableName withDefault:(BOOL)defaultValue;

Signed-off-by: Miguel M. Almeida <miguelcma@CMA-Retina.local>
Refactor: stringValueForVariable: -> stringForKey:

Signed-off-by: Miguel M. Almeida <miguelcma@CMA-Retina.local>