[C++] Implement dictionary-encoded array container

[asfimport]

This will compose an array of dictionary indices (int32 type currently per the format doc) and a dictionary array

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• [java] implement Dictionary vector (relates to)
• [C++] Implement IPC round trip for DictionaryArray, dictionaries shared across record batches (is depended upon by)
• [C++] Implement JSON round trip for DictionaryArray (is depended upon by)
• [Python] Implement conversion between arrow::DictionaryArray and pandas.Categorical (is depended upon by)

_{Note: This issue was originally created as ARROW-427. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
WIP PR: #268

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 268
#268