Spellcheck of enhancingperf.rst
Spellcheck of the docs, mostly `enhancingperf.rst`, and a sentence or two on `sparse.rst`.

* Added a short introductory paragraph on enhancing performance.
* Backticks ` `` ` around Series, DataFrame.
* Typeset variants of ` ``numba`` `, `(n|N)umba`, `(c|C)ython`, etc as **Numba** and **Cython**.
* Minor rephrasing of sentences, spelling, periods, colons etc.

Cheers. Comments very welcome :)
