Adds an additional display function + bumps core-foundation dependency version

[daniel-abramov]

This is a small pull request, basically it adds the following things:

1. Adds a missing CGDisplayCreateImage() function, so that now the user has all required functions to capture static images (it was already implemented for window capturing and now the display capturing is also possible).
2. Adds a couple functions to CGRect.
3. Updates the version for core-foundation from 0.3 to 0.4. I'm not sure if using 0.3 makes sense if we can use 0.4. For me personally it was required to change it as my project depends on several crates, including the latest core-foundation version, so when I have both core-graphics and core-foundation in my dependencies, core-foundation has been compiled twice (once for 0.4 which is the new version and once for the old 0.3 which has been downloaded and compiled as one of the core-graphics dependencies). So basically the people who use the both core-graphics and core-foundation (the last versions) would encounter the same problems (core-foundation is compiled twice and it may cause compile time errors like it was in my case). What are your thoughts on these?

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This change is 

[bors-servo]

☔ The latest upstream changes (presumably #86) made this pull request unmergeable. Please resolve the merge conflicts.

[jdm]

This looks fine. Sorry for the delay; if you rebase and increase appropriate Cargo version numbers, we can merge and publish these changes.

[daniel-abramov]

Done.

[KiChjang]

The changes here don't seem like they're breaking; why did you increment the minor version number?

[daniel-abramov]

They are indeed non breaking, that's why I incremented the minor version, not the major version. I incremented the minor version as I thought that changing the dependency versions (core-foundation 0.3 -> core-foundation 0.4) requires this change. But if it is not the case, I can revert this change and increase only the patch version.

[daniel-abramov]

UPD. I just followed semver:

Minor version Y (x.Y.z | x > 0) MUST be incremented if new, backwards compatible functionality is introduced to the public API.

[daniel-abramov]

And also I've just checked the history of Cargo.toml and noticed that the last time when the core-foundation dependency version was increased, the minor version of the crate was increased as well:
d796d9f

Another one example:
b7c2f5c

[KiChjang]

Ok, looks like the version bump in core-foundation necessitates a minor version bump. Thanks!

@bors-servo r+

[bors-servo]

📌 Commit f9f2226 has been approved by KiChjang

[bors-servo]

⌛ Testing commit f9f2226 with merge aeb8108...

[bors-servo]

☀️ Test successful - status-travis
Approved by: KiChjang
Pushing aeb8108 to master...