Improve Power (^) Scanning / Add X_SQRT()

- ignore all white space characters when seaching for the base or exponent
  in get_local_exp_part()

- add square root (X_SQRT()) to the available Excel methods
gitignore updated
Some documentation / warning updates
Code made more testable
X_FLOOR added
Merge branch 'master' of https://github.com/triple-j/XLS-XML-Formula-Expander into triple-j-master

Conflicts:
	XXLS.php