add default prefix and support prefixes with spaces
fix unit test for bad prefixes
improve doc for default locator prefix
fixes #200

Totally unrelated change I know.
typo in default strategy doc
Merge branch 'spaces-in-locator-prefixes' of https://github.com/ombre42/robotframework-selenium2library into ombre42-spaces-in-locator-prefixes

Conflicts:
	CHANGES.rst
Merge branch 'ombre42-spaces-in-locator-prefixes'