Added method for encoding a number using an arbitrary alphabet.
Removed old to_base26 function in favour of the more general one.
Reword source naming to use base62 of the first reading's exposure 
number concatenated with the base62 of the first reading's x + y values.
Changed naming to use base52 (instead of base62) of the exposure number 
to make sure the name starts with a letter.  Also added padding with 0's if 
the name is less than 7 characters.
Merge pull request #131 from drusk/robust_naming

Closes #125.