Predicates & Accessible elements

Some of the extensions I discussed here: https://groups.google.com/forum/#!searchin/frank-discuss/extensions/frank-discuss/MG3qmTLW9QI/T6fdIqu3Hx0J

### 1. Predicates
Syntax: `filter:"<nspredicate>"`
Examples:
`view filter:'tag=10'`
`view filter='text contains[cd] "something"'`

### 2. Accessibility (iOS only)
Syntax: `uiaelement:"traits"`
Examples:
`uiaelement first` - will return the `UIApplication` object
`uiaelement marked:'something'` - will return only accessibility elements marked by the given string
`view:'UITableViewCell' uiaelement` - will return accessibility elements inside all table cells
`uiaelement:'keyboard-key'` - will return all elements with trait `UIAccessibilityTraitKeyboardKey`
`uiaelement:'keyboard-key, button'` - will return all elements with traits `UIAccessibilityTraitKeyboardKey` and `UIAccessibilityTraitButton`

### 3. Accessibility identifier
`marked:` and `markedExactly:` now also check `accessibilityIdentifier`

### Note
Note that accessibility also requires an update to Frank to enable tapping accessibility elements.
Please review and test.