`RangeIndex(1, 10, 2)` is a memory saving alternative to
`Index(np.arange(1, 10,2))`: c.f. #939.

This re-implementation is compatible with the current `Index()` api and is a
drop-in replacement for `Int64Index()`. It automatically converts to
Int64Index() when required by operations.

At present only for a minimum number of operations the type is
conserved (e.g. slicing, inner-, left- and right-joins). Most other operations
trigger creation of an equivalent Int64Index (or at least an equivalent numpy
array) and fall back to its implementation.

This PR also extends the functionality of the `Index()` constructor to allow
creation of `RangeIndexes()` with
```
Index(20)
Index(2, 20)
Index(0, 20, 2)
```
in analogy to
```
range(20)
range(2, 20)
range(0, 20, 2)
```

Cache a private Int64Index object the first time it or its values are required.
Restore Index(5) as error. Restore its test. Allow Index(0, 5) and Index(0, 5, 1).
Make RangeIndex immutable. See start, stop, step properties.
In test_constructor(): check class, attributes (possibly including dtype).
In test_copy(): check that copy is not identical (but equal) to the existing.
In test_duplicates(): Assert is_unique and has_duplicates return correct values.

* enh: set RangeIndex as default index
* fix: pandas.io.packers: encode() and decode() for RangeIndex
* enh: array argument pass-through
* fix: reindex
* fix: use _default_index() in pandas.core.frame.extract_index()
* fix: pandas.core.index.Index._is()
* fix: add RangeIndex to ABCIndexClass
* fix: use _default_index() in _get_names_from_index()
* fix: pytables tests
* fix: MultiIndex.get_level_values()
* fix: RangeIndex._shallow_copy()
* fix: null-size RangeIndex equals() comparison
* enh: make RangeIndex.is_unique immutable

 * optimize argsort()
 * optimize tolist()
 * comment clean-up