Open a background non-focus-stealing window for tests on OS X.

Use false for the glfw::Visible window hint to open a background window
on OS X. This requires an upgrade to glfw 3.0.4 in order for this to
also not steal focus.
Add support for DOMString in unions (fixes #2324).
Add Performance object to Window and implement Performance::Now()
auto merge of #2364 : zwarich/servo/issue-2363, r=larsbergstrom

Use false for the glfw::Visible window hint to open a background window
on OS X. This requires an upgrade to glfw 3.0.4 in order for this to
also not steal focus.

This requires adding a new parameter to WindowMethods<A>::new.

Fixes https://github.com/mozilla/servo/issues/2363. r? @larsbergstrom
Make range generic
Use {} instead of {:?} for debug!ing in util::range
Require Range<T> to have a signed integer T

This allows for generic +ve/-ve offsets
auto merge of #2365 : bjz/servo/generic-range, r=pcwalton

Signed integers allow for generic +ve/-ve offsets.

r? @pcwalton
auto merge of #2346 : gsingh93/servo/master, r=jdm
Update rust-png for fixed color profiles in test images.
auto merge of #2366 : zwarich/servo/incorrect-rust-png-profiles, r=metajack

This will remove the

```
libpng warning: iCCP: known incorrect sRGB profile
```

spam from `make check`.