add options to show locations and change server address

[erizocosmico]

Closes #32
Closes #33

Adds the following options:

• Show / hide locations
• Use a custom server address

Also:

• Modifies the Makefile so it generates a correct API URL value for prod.

[dpordomingo]

The showLocation feature works removing the location from the tree?

[erizocosmico]

Nope, just conditionally renders the <Position> elements in the node. If we remove them from the tree there's no way to show them ever again 😱

[dpordomingo]

Yes, I know... My point is: If you hide Locations, there is no way to bring back only one of them. I'd find it more useful if it'd work like a «collapse all» toogle, so you could uncollapse certain locations on demand without having to display all at once.

[erizocosmico]

@dpordomingo but that was what it was asked...