Element methods

* First commit:
https://github.com/appium/appium/commit/4ae0cd9c28ded5b34652fe99adefe3a701c831a8 has broken methods like element.is_enabled(), I get the following exception:
WebDriverException: Message: u'ERROR running Appium command: invalid json, empty body'
and this error in the Appium console:
Error: invalid json, empty body
    at Object.exports.error (/media/shared/src/appium/node_modules/express/node_modules/connect/lib/utils.js:63:13)
    at IncomingMessage.<anonymous> (/media/shared/src/appium/node_modules/express/node_modules/connect/lib/middleware/json.js:61:29)
    at IncomingMessage.EventEmitter.emit (events.js:97:17)
    at _stream_readable.js:910:16
    at process._tickCallback (node.js:373:11)

* Second commit:
element.is_enabled() and element.is_selected() return u'true'/u'false' (string) while element.is_displayed() returns True/False (bool).
It's a fix so that all those methods return bool.