Add string accessor that works on native pandas and fletcher columns

[Uwe L. Korn]

We support string functions using fr_text and text. These only work on fletcher columns. To ease the conversion from the object / string dtype to a fletcher based string type, we should support the following:

• fr_str is an accessor that works on fletcher and pandas columns.
• fr_strx only works on fletcher columns. This is useful for users that want to be certain that accelerated operations are used.
• Drop text and fr_text accessors to be inline with pandas naming.

[Uwe L. Korn]

Fixed by #163