Finalized text of concatenating strings recipe.

Moved tests for concatenating strings into own file.
Cleaned up system for aggregating tests.
Wrote interpolating expressions recipe
Wrote incrementally building a string using StringBuffer recipe
Added recipe for converting between characters and numbers.
Added recipe for testing whether a string is empty.
Wrote recipe for trimming whitespace from a string
Added 3 more recipes.

Getting the length of a string.
Splitting strings.
subscripting strings.
Added 2 more recipes.

Changing string case.
Determining if a string contains another string.
Added 2 more recipes about using regExps.

Finding regExp matches.
Subsitituting strings based on regExps.
Added Unicode string output to examples
Tweaks. Removed print statements.