Add option change architecture and bits.

Resolves #1 so that architecture can be specified and also bits.

![screenshot](http://i.imgur.com/3vJlWUy.png)

Also implemented:
* Show bits menu entries depending on architecture
* Disassemble menu item for Hex view (useful for right click, select all and then disassemble)
* Allow most options (eg. asm_syntax) to be used even when not analyzing
* When analyze option is deselected clear other unused options*.
* Lowercase/uppercase instruction option used also for Hex View

*- in current version there's a bug that if you set analyze mode, select some options and then switch back to non-analyze mode those options will still be set.

Implemented it in `ui/radare_core.py#get_plugins()` with catching exception. No need to check which version used.