Revert grep to original functionality with strings converted to regex's....

...  Add Mocha#find() which escapes the string.

We have a bunch of grep-strings which we pass via URL using a script.  With commit b00b6f54, the Mocha#grep() behavior changed so that the grep strings, which were previously treated as regular expressions, are now escaped.  This broke our grep-patterns.

To me, the word "grep" implies regular expressions, and it is occasionally useful to use regular expressions when selecting tests/suites.  I've reverted Mocha#grep() to the original functionality and implemented the new functionality in Mocha#find(), adding support for a new URL variable "find" which uses the new function.

You may not want this behavior in the main tree, but I thought I'd pass it along.  Feel free to reject.