docs(directive): Clarified and cleaned up directive guide

- corrected terminology about how directives use `require`
- added more variations to the DirectiveDefinitionObject
- removed some slightly superfluous text

docs(directive): Minor correction to example to avoid bad practice

Anchor tags should use `ng-href` instead of `href` for interpolation.

docs(directive): Supplementing DDO description

DDO = Directive Definition Object
Tweak recommended here:
https://github.com/angular/angular.js/pull/2888/files#r4664565
fix(numberFilter): always convert scientific notation to decimal

Previously, the number filter would format small and large numbers
as scientific notation. It now uses toFixed() to ensure that all
requested digits are shown.
fix(ngRepeat): handle iteration over identical obj values

Modifies default trackByIdFn to factor both key and value into hashKey
for non-array primitive (i.e. index not provided) values

Closes #2787
Closes #2806
fix(form): pick the right attribute name for ngForm

Closes #2997
feat(ngRepeat): add $even and $odd props to iterator
fix(ngMobile): prevent ngClick when item disabled

 - the ngClick attribute was always triggered, regardless the ngDisabled/disabled attributes
 - we now check the DOM disabled status before triggering the original click event

Closes #3124
Closes #3132
fix(select): don't support binding to select[multiple]

changing the type of select box from single to multiple or the other way around
at runtime is currently not supported and the two-way binding does odd stuff
when such situation happens.

we might eventually support this, but for now we are just going to not allow
binding to select[multiple] to prevent people from relying on something that
doesn't work.

BREAKING CHANGE: binding to select[multiple] directly or via ngMultiple (ng-multiple)
directive is not supported. This feature never worked with two-way data-binding,
so it's not expected that anybody actually depends on it.

Closes #3230