Prevent PHP54 default obj warn and return value

`$bt->controller = ` will throw 'Warning: Creating default object from
empty value' in PHP5.4+ if block handle is not found.

In PHP5.4- this will return a instance of StdObject not BlockType if
block handle is not found.

Can cause issue with `if(BlockType::getByHandle('foo')) {` package
installer idiom.

* Sets return value to `false` to remain consistant with -1 cache value
`return false` if block handle not found
* Eliminates uneeded `else` statements that follow `if` statements which
end in `return` statements
* Corrects typo in phpdoc comments
* Corrects phpdoc @return to mixed
Merge pull request #905 from mkly/bugfix_blocktype_getbyhandle_cache_and_php54

Prevent PHP54 default obj warn and return value