Refactoring extension known / install codes. and NOW also support bitbucket.

Again, I have refactoring extension known / install codes  that make us register new extension hosting services more easily.

And for example, a new `bitbucket` hosting has added.
Now we can using `phpbrew` to install extension from `pecl` , `github` , `bitbucket` or whatever ...

Known example:
```
phpbrew ext known xdebug    // from pecl.

phpbrew ext known https://github.com/phalcon/cphalcon   // from github

phpbrew ext known https://bitbucket.org/osmanov/pecl-event    // from bitbucket
```

install example:
```
phpbrew ext install xdebug    // from pecl.

phpbrew ext install https://github.com/phalcon/cphalcon   // from github

phpbrew ext install https://bitbucket.org/osmanov/pecl-event    // from bitbucket
```


Signed-off-by: Rack Lin <racklin@gmail.com>
just a nit: wonder if we really need `&` (pass by reference) here...