Support nullable types, including primitives, enums and structs. Tests included.
Nullable type support based on https://github.com/chipdude/litjson/commit/eaa84eed6e0d876b962c5b33b4fc4a01b4fdbd3b
Update JsonMapperTest.cs, changed ExportValueTypesTest, ImportValueTypesTest

ValueTypesTest has ulong type attribute, but it doesn't support ulong max value limit.
Update JsonReader.cs

For reading unsigned long (UInt64) type data, 
added some code to pass ValueTypesTest's ulong with max value.
Add make variable for specifying pkg-config

Mono ships its own pkg-config which knows about all of its tools, such
as nunit-console2. It doesn't really play nice with Mac and Homebrew, so
many leave it out of their path.

With this, you can now run tests from a terminal on Mac like so:

```
$ make MONO_PKG_CONFIG=/Library/Frameworks/Mono.framework/Commands/pkg-config test
```
Change MONO_PKG_CONFIG to PKG_CONFIG and note usage in README
Getting pull request #6 ready

- Added NEWS item
- Code style fixups and fixed warning in JsonMapperTest
Merge branch 'michaelbartnett-support_nullable_types'
Merge branch 'patch-1' of https://github.com/MickeyKim/litjson into MickeyKim-patch-1
Merge branch 'patch-2' of https://github.com/MickeyKim/litjson into MickeyKim-patch-1
Getting ready pull requests #13 and #14
Merge branch 'MickeyKim-patch-1'
Merge branch 'add-make-var-for-pkg-config' of https://github.com/michaelbartnett/litjson into michaelbartnett-add-make-var-for-pkg-config
Getting ready pull request #27
Merge branch 'michaelbartnett-add-make-var-for-pkg-config'
No longer track the .dll
Minor edits to README
Preparing 0.9.0