Ion reader builder

[R-maan]

*Issue #28 *

Adding an IonReader Builder with IonValue as input parameter.

[tgregg]

Please rebase these changes onto the latest revision of master so that the diff doesn't include code that has already been merged.

[R-maan]

Done. Sorry I didn't noticed it.

[tgregg]

The port basically looks good, with a few comments below. However, we really need tests so we can verify it works.

In ion-java, the test that's directly relevant is TreeReaderTest. However, additional coverage is added in various tests parameterized by ReaderMaker (see ReaderMaker.FROM_DOM).

In ion-dotnet, we have ReaderTestCommon, which defines some common tests for readers. It looks like you could achieve roughly the same effect as ion-java's ReaderMaker (albeit without the fancy built-in test parameterization) by constructing UserTreeReaders over the appropriate input data via IonReaderBuilder and invoking a corresponding ReaderTestCommon test.

[tgregg]

Closing in favor of #51