Add g_str_is_ascii()

Add a function for checking if a string is pure ASCII.

https://bugzilla.gnome.org/show_bug.cgi?id=709753
Add simple string "search" function

Add a pair of functions to make it easier to do simple string matching.

This will be useful for use with things like GtkSearchBar and will also
be the basis of the searching done by the (soon to appear)
g_desktop_app_info_search()

https://bugzilla.gnome.org/show_bug.cgi?id=709753
Add testcase for search utilities functions

https://bugzilla.gnome.org/show_bug.cgi?id=709753