ENH: Upgrade Excel I/O module to use OpenPyXL 2
[OpenPyXL](https://pypi.python.org/pypi/openpyxl) v2.0 was released on May 13th, breaking backwards compatibility in some parts of its API, including those upon which pandas depends. Notably, v2 changes [the style API](https://openpyxl.readthedocs.org/en/2.0.2/styles.html) and makes `Style` objects immutable. This incompatibility is evident at 0.14.0-rc1 in [`pandas.io.excel._OpenpyxlWriter._convert_to_style`](https://github.com/pydata/pandas/blob/v0.14.0rc1/pandas/io/excel.py#L600) and the associated test case, [`pandas.io.tests.test_excel.OpenyxlTests.test_to_excel_styleconverter`](https://github.com/pydata/pandas/blob/v0.14.0rc1/pandas/io/tests/test_excel.py#L1131).

In addition to teaching `pandas` how to use the new API, the project may wish to retain support for the OpenPyXL v1 API, necessitating the design and development of a compatibility shim.
