Eliminated several Xcode warnings.

Removed a stray semicolon from `TFHppleElement.m` to eliminate this warning:
```
semicolon before method body is ignored [-Wsemicolon-before-method-body]
```
Moved instance variable declarations from headers to extensions in the implementations to clear this warning:
```
declaration of instance variables in the interface is deprecated [-Wobjc-interface-ivars]
```
Removed `strong` attribute from `data` property in `TFHpple.h` to silence this warning:
```
property attributes 'readonly' and 'strong' are mutually exclusive [-Wreadonly-setter-attrs]
```