Drop native extension

Closes: https://github.com/CocoaPods/Xcodeproj/issues/167

The performance hit is noticeable:

###### master

```
Initialization from a file
    ✓ sets itself as the owner of the root object (33 ms)
    ✓ can be loaded from a plist (32 ms)
    ✓ can regenerate the EXACT plist that initialized it (45 ms)
    ✓ doesn't add default attributes to objects generated from a plist (31 ms)
  #Save
    ✓ saves the project to the default path (13 ms)
    ✓ saves the project to the given path (6 ms)
    ✓ can open a project and save it without altering any information (535 ms)
    ✓ escapes non ASCII characters in the project (6 ms)
    ✓ uses xcproj to convert the project to match Xcode output (5 ms)
    ✓ skips the xcproj to convert the project to match Xcode output (5 ms)
```

###### This patch 

```
Initialization from a file
    ✓ sets itself as the owner of the root object (478 ms)
    ✓ can be loaded from a plist (482 ms)
    ✓ can regenerate the EXACT plist that initialized it (951 ms)
    ✓ doesn't add default attributes to objects generated from a plist (472 ms)
  #Save
    ✓ saves the project to the default path (51 ms)
    ✓ saves the project to the given path (45 ms)
    ✓ can open a project and save it without altering any information (2.1 s)
    ✓ escapes non ASCII characters in the project (28 ms)
    ✓ uses xcproj to convert the project to match Xcode output (12 ms)
    ✓ skips the xcproj to convert the project to match Xcode output (11 ms)
```

Please lock the CFPropertyList and libxml-ruby gems to the exact versions that come with OS X. On 10.10 (which only has Ruby 2) those are:

* CFPropertyList-2.2.0 (so this one should be used)
* CFPropertyList-2.2.8
* libxml-ruby-2.6.0

Please check which versions come with system Ruby 1.8.7.