DOC: Cleaned references to pandas <v0.12 in docs
There is a lot of references to in the docs to when exactly some change occured.  For newer changes this is great, but there comes a time when such references only disturb the reader rather than help him, as the versions referenced become so old, that they become noise rather than help.

I've cleaned up references up to and including v0.11. 

IMO I could have gone higher (v.015?), but can do that in some later round.

Some issues I would be glad for input on:
* In ``gotschas.rst``, there is a sentence "As of pandas 0.11, pandas is not 100% thread safe." I haven't altered this, but I presume this still is correct in the newest version of pandas? Then IMO it should be changed to reference a newer version or simply to "pandas is currently not 100% thread safe."
* In ``io.rst`` there is a sentence "0.10.1 of ``HDFStore`` can read tables created in a prior version of pandas, ...". I'm not even sure the "0.10.1" references the version of pandas or a HDF library and I left it alone. The paragraph also discusses backwards compatability, which makes it somewhat relevant to keep around, even if it's an old change.