Dropdown Select Metadata Type

This PR adds a new custom metadata field type option called "picklist" (could just as easily be called a "dropdown"). When a user creates a new custom metadata field and choose the "picklist" type, she must also input one or more newline-separated picklist values ("items"). Like this:

![](https://cloud.githubusercontent.com/assets/1349496/2696991/04388646-c3e9-11e3-9c5b-eb2336f5b833.png)

The result is a new `select` element on the post edit page whose options are the user-enumerated picklist values from above.

For certain cases where it's useful to have a custom metadata field with a finite list of possible values, using this picklist/dropdown is preferable to using the current "Text" option for the sake of data cleanliness.