Added tests for string recipes.

concatenating strings
interpolating expressions
converting between string characters and numerical codes
determining if string is empty
trimming whitespace from a string
getting the length of a string
subscripting a string that contains symbols
splitting a string
changing the case of a string
determining whether a string contains a substring
finding occurrences of a string inside another string
substituting strings based on regExp matches
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.
Tweaks and formatting changes
More tweaks
Corrected typos
Substantial rewrite of all the string recipes
Numerous minor tweaks
Many tweaks. File renames
Numerous tweaks
Added file: book.asciidoc