Changed type of Location.ID from int to string.

LocationsService is never tested against real call to Instagram API.
It turns out the ID field has string type instead of int. Since this
breaks compatibility, I'll release 0.3.
Bump version to 0.3.

This also updates the default user-agent string to package name
plus extra space and lib version. The reason is so that Instagram
can easily reach out the lib as 'go-instagram' doesn't provide
good result on search engine.